I started using an "engineering notebook" for things like that. They are called other things in other lines of work or school (lab notebook, captain's log, ...). In an actual engineering notebook, you would use a sewn binding where you can't add or remove pages, the pages are numbered, you have an index in the front, entries are made in strictly chronological order, you'd have someone else witness and sign pages, draw a diagonal line across empty areas so you can't remove things, only cross things out with a single line, etc. All that because you might need to prove you invented something and other legal CYA reasons. Most of these requirements, you don't need for personal projects, though. But the things that are useful: keeping track of projects, what settings you've tried, what the results were, and so on. That way you don't have to reproduce it from scratch again later. "What settings did I need use to create the gerber files in kicad for manufacturing? How did I set ___ in the oscilloscope? What parts did I use to make that thing? What things did I wish I had checked last time? Where did I put those files?" You can also print things out and tape them in for reference.

MEMORY.md is your friend.