Another weirdness was that for the first couple of years of the Mac's existence you had to have a Lisa if you wanted to write code for it. The Mac had so little RAM that it couldn't run a Pascal compiler. For this reason, when I bought a Mac in 1984 I also bought a Lisa with a huge 5MB (!) hard drive.
You bring up a great point though: Whatever happened to LisaOS? Did anybody archive the source anywhere or did it completely vanish?
https://info.computerhistory.org/apple-lisa-code
https://github.com/azumanga/apple-lisa
Just two years ago Apple released it under a very strict non-commercial use "be careful how you look at this" license, to the CHM:
https://computerhistory.org/press-releases/chm-makes-apple-l...
I briefly looked at it, it's a pile of Object Pascal and M68k asssembly. I haven't looked to see if anybody has managed to make it compile in any kind of available-today compiler yet.