Ironically, SQL Server AFAIK in order to run on Linux uses what basically amounts to a Microsoft reimplementation of Wine. Which always makes me wonder if they'll ever get rid of Windows altogether someday in favour of using Linux + a Win32 shim. I think there are still somewhat strong incentives nowadays to keep NT around, but I wouldn't be that surprised it this happened sometime down the line.

It's a Windows container. It runs the NT kernel and a few minimal other things. The closest would be the Nano Server container

AFAIK it's more like a reimplementation of NT APIs in userspace - aka basically Wine with extra steps, or Linux UM. There was a slide deck going around about Project Drawbridge, here: https://threedots.ovh/slides/Drawbridge.pdf