For example I create 1 cube and extrude as much and as many things as I want out of it, but then I need to add a new cube to add a simple thing which couldn't be easily extruded from current mesh, so I do it...
Does that new cube get exported as separate mesh or are they joined into one mesh when exporting? And if it's not the latter, what would be the simplest way (if possible) to join them and delete faces which would be obstructed?
I've never used blender before (nor any other modelling software) so I have no idea how most stuff works in here.