I started my first attempt to contribute to the Postgres core codebase in June, and after a few review cycles the contribution was committed!

https://github.com/postgres/postgres/commit/069d33d0c5a021601245e44df77a0423ddd69359

It's not a huge change, but I learned a bit on how to build & test dev versions of Postgresql. I also learned about the contribution process and the high standards the committers hold is quite promising!