How does Unity compile 3D Models?

when you build your game, how does unity compile all the different 3D files (.obj, .3ds, .x)? i just wanna know if unity compiles it all into one object file or it just reads all different types on the go

and is there a source where i can look this up? i couldn’t find it.

No, there is no source available to the general public.

Unity does not store meshes as obj/fbx/3ds/etc, it loads these formats in Editor and then stores them in binary representation. This representation is something like you see in Mesh class (Unity stores data in a bit optimized format, and gives simplified access to users). So when you build your game no source files are put into it - only final representation.