By default, there is a grid in unity and the y axis is perpendicular to it. Did developers do it with a certain aim?
For example, if I rotate camera so that it renders XY surface will I have problems with engine functions later? Will physic functions, like gravity, work, and will it be possible to import 3d models?
If it doesn’t matter, can I set a default axes direction for a new game object, not to rotate it every time?
You can have “down” be whatever direction you want. You can modify the default direction of gravity wither through a script (Physics.gravity, if I recall correctly) or via the Editor by clicking Edit → Project Settings → Physics and modifying gravity’s component values there.
If you make your models following Z-up, then they’ll import as such into Unity, presuming your file exporter doesn’t flip them (which it shouldn’t, but I’ve run into that before).
If I am understanding your question the gravity will not work the way you may want it to. If you bring in a model it will keep the same position repetitive to the x,y,z axis of the modeling program, however the gravity by default will still use Y as up.
The Y axis is set as a vertical line that runs up and down in every bit of software as that is how it correlates mathematically. The X, Y and Z are all based on mathematics grids so if all the physics are written for that grid, it would be easier to just make your game assets with your end goal in mind rather then try and rewrite Unity’s gravity in your game world.