Which one should be the "up" axis

Which one shoud be the “up” axis when making a platformer game and in a game where I view the map from above? (z/y?) Thanks!

There is no “should” - you just choose your coordinate system and make sure that things are consistent.

I find that the convention seems to be that with 2D applications -Y is up and in 3D applications +Y is up. Again, you can just make up whatever you want, just stick to a convention.