[pjmlp]

Win16 and NT branch don't share almost anything.

Probably the only port was Win32s, which was a subset of Win32 backported to Win16.

Win16 stuff runned on a VM like environment, WOW.

https://en.m.wikipedia.org/wiki/Windows_on_Windows

[hedgehog]

I'll admit I haven't seen the NT source but from a user perspective NT's UI (widgets, progman, etc) looked and functioned near identically to Windows 3 so I've always thought those bits were ported over. I'm aware most everything under that was replaced. To my eye that was one step in a well-executed evolution up from legacy Windows towards the eventual convergence in XP.

[temac]

Yes the GUI was mostly ported from consumer Windows to NT. First the 3.1 GUI, then 95.