There are a few things about computer hardware that I'm wondering about, but wouldn't fit in my topic about building a computer, so I am wondering whether anyone could answer these questions:

* What is the closest AMD equivalent to the Intel Atom x5-z8350? (Ideally, it should be possible to buy a $100 computer with this CPU.)
* What are the closest AMD and Intel equivalents to the ARM Cortex-A72? (This is the CPU in the Raspberry Pi 4.)
* What exactly is a mechanical keyboard, and why would I (not) want one? How would such a keyboard compare to the one I'm using now, which I believe I got in 2003 or so?

I may have more questions later.

https://www.cpubenchmark.net/ has a lot of comparison info on different CPU's.

Mechanical keyboards are more durable and faster in their response times. Unless you are the type to pound on the keyboard during a game I highly doubt you will need it.

It doesn't seem to have ARM CPUs, however.

That’s not entirely true. Mechanical keyboards can be more expensive, but not by much if your careful. Also they do not have to be noisy. For instance, I use:
https://www.amazon.co.uk/Logitech-G413-Mechanical-Gaming-Keyboard-Romer-G-Through-UK-Layout/dp/B06XPZQWZ8/ref=sr_1_1?keywords=steelseries+keyboard+g413&qid=1579770585&s=computers&sr=8-1
It’s a great keyboard, reasonable price considering the extra durability, and you don’t hear the keyboard. It’s also extremely responsive.

A final note, mechanical keyboards can generally pickup more than one leper’s simultaneously. Each one is it’s own conduit, where more basic keyboards have multiple keys,or the whole keyboard tied to one conduit meaning you cannot press more than one at a time and it respond. So if you play games, which maybe require multiple keys at the same time, mechanical is way better. Response time should also be better.

If you want study a bit about mechanical keyboards, 'Mechanical Keyboards Inc.' has nice info-pages:

Learm main menu
What is mechanical keyboard(comparison with typical rubber dome or membrane)
Vast amount of switches used in these keyboards with specs
Terms used with keyboards

Like others have said, mechanical keyboard gives "feedback" when you pressed the key, membrane/rubber dome-ones might feel "mushy" (varying degree) when pressed so there's no good feedback did you press key enough to register or not.

Good computer stores should have demo keyboards so you could check if they are something you want to invest.

I just got a fancy $100+ mechanical keyboard for Christmas. It's kind of cool, has a more tactile and satisfying feel to the key presses, but it's FAR from needed. Also me personally, I don't really use all the added function keys because I'm so used to normal PC game controls (M for map, etc.). Maybe that'll change someday but I dunno...

In short, if you made me go back to my $20 keyboard I wouldn't care much.

Benchmarking ARM vs x86 is... problematic. You can do ARM vs ARM or x86 v x86 easily enough but the architectures and (generally) usage profiles are completely different- kind of like comparing a scooter vs a family car; you can compare them but it's nowhere near as good/ fair as comparing car v car or scooter v scooter and usually ends up being slanted depending on who is doing the tests and what their criteria are.

There isn't really an x86 equivalent to a Raspberry Pi. There are some low power x86 SoCs/ embedded solutions from both Intel and AMD, but there's a very good reason why the Pi uses ARM, that sort of low cost low power system is what ARM does great at and is why it's dominant in phones too; but Intel despite spending billions is still way behind in its efforts (and AMD hasn't even really tried to compete in that section).

As far as I'm aware AMD has not had any processors in the ultra-low power segment for a while now, so there's really no equivalent out there. Their G-series processors can be found on some embedded systems, but I'm not sure if they go as low as x5-z8350's 2W SDP. We're comparing apples to cherries here, but if you must compare the Cortex-A72 to a x86 processor, I guess you've answered this yourself: Intel Atom x5-z8350. A mechanical keyboard, as opposed to the more common membrane keyboards usually found on laptops, features different types of key switches which usually have a deeper travel distance and a more solid overall feel. Some people would describe them as being closer to a "typewriter experience", if that makes any sense. There are other advantages to them as well, inherent to their design, such as somewhat lower response times. They're also noisier and what I'd call more "clunky".

I don't buy the argument about mechanical keyboards having lower response time. Electrically, there is no need for it to be so. It all boils down to how the controller is programmed. Likewise, there's no reason why a rubber dome key should not last as long as or longer than a mechanical switch. It's a very solid construct, well protected with not much moving parts, and in my experience suffers less from oxidation (which can be an issue with mechanical switches). I have never had a rubber dome keyboard break due to age or excess use, but I have mechanical cherry mx switches that are having contact issues, especially if left unused for some time.

The main difference between a mechanical and rubber dome keyboard is that a rubber dome makes contact -- and signals a key press -- only when you "bottom out." It's not very comfortable for fingers to bottom out, but you might not notice if you're used to it. Mechanical keyswitches generally conduct at a point partway into its travel, and switch design can incorporate a tactile bump at the activation point. You'll also find very linear and very clicky switches.

Why would you not want one? Pretty much price is the only reason I can imagine. Many switches are loud, but I hear there are quieter switches around, and people also mod them with O rings that are supposed to dampen the clackity clack.

Also, choosing the best switch for you might be easier said than done. And IMO not all switches are good. So you might spend more money on swapping switches.

EDIT: attached photo of my current keyboard. It's got kailh box white switches, and it's loud. 200 usd + tax & customs. One day I will build my own keyboard.

AMD doesn't compete in every areas with Intel. There is no direct competitor With the Atom X5 line, because they are so freaking cheap. It's impossible to compete to a 100$ product wich includes a whole motherboard with 2Gb memory, power supply and a full Windows 10 license (like those HDMI sticks).

Back in the older Atom line without out of order execution (like the n450), AMD had better CPU's for the price (C-60 and E-350) but as it has been for a very long time, all AMD laptops are crippled and considered inferior (Intel is still the premium brand and AMD the 2nd choice), using a slow hard drive, worse screens etc... Don't know why and don't care really.

At this time, except the embbeded stuff there are only non-Ryzen laptop CPU's like the a4-5000 or the E2-3800, wich are slow quad-cores, like the X5-Z8350 but compete in a different market. Non-Ryzen stuff are still stuck at 28nm wich is 2010 tech?

I've read about some cheaper SBC based on AMD but nothing in the 100$ range. Searching can find some stuff but this is way out of the 100$ range in performance, more like 300 to 400$.

AMD isn't a direct competitor to Intel, wich makes a very broad range of products in different areas and the only reason AMD desktop CPU's are so good at the moment is because Intel is stuck on tech progress, so they say...
Neither the less, AMD Ryzen line is a very solid choice (stay way from other line) at the moment and for a custom built computer they are selling like hot pizzas for a reason! Intel has the X5 Atom (and X7 as well) line or the newer n4000 and n5000 line Celerons (wich direct replaces the Atoms). X5 Atoms can run natively Android, not sure about the newer Celerons.
AMD has none.

If you want to get a rough estimation of x86 cpu performance this is a good resource.Keep in mind Cinebench is a renderer very optimized to scale well with more cores. That said, a CPU with more core may score better but doesn't give the best allround, day-to-day performance.
No Arm though.

My experience: a lot of rubber dome keyboards become 'sticky' with age (but not all of them).

But that's just MY experience.
(when reading specifications - or even in-depth articles, mechanical keyboards usually have a longer lifespan)

Don't personally like mechanical keyboards but they tend to be higher quality. They make a "click" noise anytime a key is pressed, just like a mouse (not a perfect comparison but the way they work is similar).
A good rubber switch keyboard can be better than a cheap mechanical keyboard. Like anything "gaming" related, is just another market name, with comes on black, red and with rgb leds..

I like keys with short stroke, like laptop keyboards and those HP and DELL that come bundled with desktops, gladly many people throw them away :D

Some make a click, others don't. There's tons of different kinds of switches and mods.

My old mechanical keyboard has a built in speaker to make a click, because the keys themselves aren't very clicky (though they're still noisy)

It's almost like comparing species that breath oxygen with species that breath methane or sulfur, they all have their own "world rules".

In this case, RISC (Reduced) based CPU's and CISC (Complex) based CPU's is rather alien to each other in terms of ecosystem. Low-power ARM RISC instruction set (Broadcom, MediaTek, Qualcomm Snapdragon etc) has relatively little in common with high-power CPU's like the x86 instruction set (AMD and Intel).

TVs, clocks, routers and mobile phones has other needs than a full blown desktop/server CPU/SoCs, direct comparing is not easy, but they are certainly getting more powerful each year.

https://www.electronicshub.org/risc-and-cisc-architectures/

https://en.wikipedia.org/wiki/Comparison_of_instruction_set_architectures

AMD consumer equivalent to Intel? Perhaps one or some of the A4/A6 cpus, but 100 buck PC? No.