Not exactly sure how this was pulled off, but this is a pretty cool hack. Apparently, this is running via x86 emulation. Just imagine how this would fly if Apple opened up the ability to run the M1 native version of macOS Big Sur on an iPad.
The video is long, mostly because of how slow the process is, so best bet is to scrub through it and look for screen changes. There are major changes at about 3:48, 5:46, and 18:33, just to get you started.