Which of the 4 are you talking about?
A taco is made from a soft corn tortilla (the American imitation came from the way people pick up tacos normally, it makes that sort of shape, leading to the selling of the hard-shell variety where it looks like that already) piled up with...pretty much whatever you want.
A tostada is basically the same thing, except on a fried tortilla.
The Mexican pizza uses two tostada shells, putting the meat between them and throwing everything else on top with a liberal sprinkling of cheese.

The tortilla is to Mexicans what bread is to Americans/Europeans. So long as you have a piece of it, you can make practically anything.

Wait, how did we go from Descent to Mexican food?

Not the first one, as I said ;)
How you make the 'bred', the tacos? I imagine you use not normal flour, or?

I offer no explanation other than the following:
https://media1.giphy.com/media/12cTRwNAKp5z5m/200.gif#15
The taco is the thing with all the meat and whatever, made using a tortilla.
What you're looking for is how to a tortilla. Honestly, most people just buy them. You could make your basic flour ones with a mixture of lard, salt, baking powder, and plain flour, but for the authentic corn ones, you're going to need what's known as masa harina. Mix that with some water and fry it in a pan (the secret is to make sure the pan's made of cast iron).

Pizza Time

There are soft tortilla, like the ones you normally use with a hot dog, only a little thicker, bigger and made by potatoes/flour/spelt or in combo. And there's the hard one, made by corn, sunfloweroil and salt. (tortilla chips or shells).

Taco or Fajita is just spicy meat with lots of vegetables, cheese, and sourcream in a tortilla. Hmm, can't beat that! :-D

Also, here in Norway there's something called Grandiosa Nacho Pizza (THIS is what I was thinking of, with extra spicy stuff, when I saw that animated gif of yours, zeogold :) )

okay same way as to heat up a pizza ;) (gylden og smeltet makes me hungry again)
But as you said you need some eeeeeextra stuff on those ;)

That could be a book title from one of the original devs after so many years..."How I went 360 degrees from Descent to spicy food".

XD

lol, oh yeah - those pizza's too skinny for my taste :-D