When I worked as an employee I contributed mostly just with docs fixes or bug reports (and not that often). I think it's mostly a mindset of seeing an issue yourself and noticing that others probably also are confused or misled by something so you go and report it!
Now that I'm working full-time on open-source and my own company I can contribute more easily to projects like upgrading SQLite source in a few bindings libraries [0], [1], [2] when 3.38 came out.
If anyone is interested in contributing to open-source and wants a bit more guidance though I have a number of good "first timer" projects related to data tooling for [3]. Only expectation is that you have some experience with Go. Join discord.multiprocess.io, go to the #dev channel and say hi!