Any individual project that made me learn cool stuff...

Maybe the kernelcheck project? It's a shell script that I wrote 2 years ago (it's still on my GitHub but the code looks kinda horrible tbh), and it really made me respect the stability of package managers, and the effort that package maintainers must put into it. Even a single package (the kernel) that you have to maintain the integrity of the .config for (the configuration file that tells you what options to compile in, as a module, or not at all) while on every new minor release, the config changes ever so slightly.. at some point I figured that I'd really need to do those compilations manually, to be able to supervise (and if necessary adjust) it in real-time. The ability for distribution maintainers to do this for thousands of packages.. it boggles my mind. Respect!