The source code for the Apple Lisa is available. It's not open source though:

https://computerhistory.org/blog/the-lisa-apples-most-influential-failure/

https://arstechnica.com/gadgets/2023/01/revisiting-apples-ill-fated-lisa-computer-40-years-on/

and here's the emulator:
https://github.com/rayarachelian/lisaem

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* Origin: War is Peace. Freedom is Slavery. Ignorance is Strength. (21:3/102)