MINIX also one of the few affordable Unix-like operating systems you could run on your own PC, and it came with full source (except for the compiler)
Linus was running MINIX when he created Linux.
And finally there the famous discussion by Linus and Andy Tanenbaum (the creator of MINIX) about how operating systems should be structured.
https://www.networkworld.com/article/3236064/minix-the-most-...
For early versions of MINIX the goal was teaching. Many limitations were left in place, both to keep the system simple and leave space for assignments for undergraduate students.
Many people in the MINIX community went to the Linux community because Andy Tanenbaum would usualy reject patches and Linus would welcome them.
Way later the MINIX3 project was created as research project. It has many cool ideas.