Back in ~2011 (i was still in high school, not even started uni), I used to work on CyanogenMod, later known as LineageOS. We would make custom aftermarket mods to Android OS. The Android OS famously is a "copyleft" style project which is not developed entirely open source but Google would work on a new version year long closed source and then dump the whole 1 year history of commits when a new version came out. So if you had forked say Android 2.2 Froyo and made a ton of changes, then a year later Android 2.3 Gingerbread comes out, the android/framework master branch would get a sudden dump of 5k commits, which might have huge conflicts with all of your work over the year. I've fixed merge conflicts which would span 4-5k lines across hundreds of files. It would sometimes take a whole week or even multiple weeks to do it. You'd even commit half-done work (partly conflicts solved, partly with the <<<<<<<< HEAD lines) so that others can continue from there. It was both incredible learning, doing all that, but at the same time, immensely painful grunt work, which I'm happy to not have to do anymore.