Back in the day, it was a big deal (at least for me) to explore the TRS-80 operating system in ROM. No open source there, but many painstakingly disassembled the code and published the results (not always completely accurately). Here is one example.
With today's standards, it's a little difficult to explain to younger folks, but it was truly exciting and fun. Plus, it was a great way to learn Z80 assembly beyond the few books...."Ohhhh that's what their doing there"
and then you would use it in your own code and you could do all kinds of cool stuff.
I know such challenges still exist, but somehow they do not seem as much fun.