Well, there is Jira with the tickets of course, but when I’m working actively on a ticket I do two things:
Text file in Notepad++ (something with custom markup so I have regions) or recently OneNote (just so this crap is backed up), write my hours down before putting them into the system, current ToDo things, notes, etc. till the ticket is through. Anything actually worth keeping gets puts into the Jira ticket as comment on close
While working on the code and I need to do something before committing it I use comments. Like // todo# do x, I’m using todo# because if I text search the code base for just todo I’ll find a hundred entries… adjust to whatever is unique to you so you find your own comments again
It’s a shit system, but has worked for me. I also write everything down so I don’t forget it, that has saved me several times.
In the past I even took my text notes and put them into an archive (with the Jira ticket linked) just if an old issue pops up and I still have my thoughts from around that time. But nowadays I do less of that because code changes too fast anyway.