Even if Windows is open sourced (which would be pretty cool just to peak in there), there’s no way on Earth you’d be able to compile it. I heard a full build takes over 18 hours on insane hardware, talkin 64+ cores.
How long do you think it takes to compile a full Linux desktop distribution like Debian or Arch Linux? Small communities manage to do this regularly. Maybe not so nice on an individual laptop, but there are ways.
I compiled GCC from source recently - it took several hours on an 8-core Xeon with 64GB of RAM.
Lots of projects take a long time to build, so it's not a show-stopper by any means. I imagine you could also mix precompiled parts with parts you compiled from source too.
It's going on fifteen years since I had anything to do with the Windows build, but that sounds...excessive. You could certainly take the afternoon off, though.