A couple questions:
is it possible to re-bind the keys?

how to use a key on a door?

I remember enjoying this a lot back in the day, but it's pretty impenetrable now haha

I don't think there's a way to re-bind the keys, but you can either left-click on a menu item, or you can use the F1, F2, F3... keys to access the different menu items, like "use" or 'talk."

There's also a mouselook patch (https://github.com/JohnGlassmyer/UltimaHacks#ultima-underworld-the-stygian-abyss) which I highly recommend that creates some new keybindings. [E] will "use" an object, like a door or a person (that allows you to talk to a person) and [Q] allows you to look at an object. The [`] key will allow you to switch between having your mouse centered on the screen where moving your mouse will move your POV, and moving your mouse around to interact with inventory items. There are some other very useful keybindings that come with this patch as well. It takes a little effort to install the patch, since you'll need to unarchive the game.gog file, patch the executable, and then compress all of the game files back into a new game.gog file with the new, patched, executable. But it's well worth it.

To use a key on a door, you'll need to "use" the key, which will change your mouse icon to the key, and then you can move the key icon on a door in order to use the key on the door.

RTFM if you have any issues. The Ultima manuals back in the day were super essential reads! Very difficult to play without reading the manuals.

Thanks, I'll give that a try next go-around.

And yeah, I had a gander through the manual but couldn't find anything in regards to keybinds. Now I know why!

You can also just use the standard cursor. Left click on the key and then right click on the door. This uses the key as well, without having to use the "use" icon.

Right clicking something in the environment, holding the right mouse button, moving the mouse and then lettig go of the right mouse button works as a general "use" command. This way you can interact with shrines, talk to NPCs, open doors, drink from fountains etc. You can drag and drop items from the environment or your inventory as well. Left clicking on your equipped weapon activates combat mode. You don't really need those icons on the left side of the screen at all.