KEEP NOTES. Write everything down. Write down what you were thinking about, how you felt about decisions, things you tried for bug fixes. you never know when this stuff will be useful to you again. I keep really basic markdown notes in a git repo, roughly one file per day with an ISO date name, but some things I give a separate file name (still with the iso date, but with some descriptive words) for things that might…
The first thing I do for every project is set up a development blog. Hugo + a nice free theme + Gitlab Pages + Netlify CMS. Everything is contained in Gitlab and I can keep the Pages site private and only accessible by members of the repo. It is less of a daily log and more exploring "issues" - basically anything I took time to research and figure out. I'm very adamant about recording details like dates, version numbers, context, code snippets, commands, and URLs of resources used. It all comes down to asking myself, if I see this issue again in the future, what information do I need to easily solve it without opening 40 browser tabs?
At first it takes some effort to circle back and write the post after you solved the issue, but after a while I found myself writing the post concurrent with writing my code. The Netlify CMS isn't the most beautiful UI but being able to just write markdown in a browser window eliminates having to keep a git repo locally. Organizing with categories and tags makes referencing published posts easy and if you want to get fancy you can integrate Algolia for search.
Honestly, having a project dev blog has saved me so much time since I don't need to keep every detail in my head anymore. Being able to go back and read about why certain decisions were made or how I overcame complicated problems is absolutely invaluable. And of course having this rich documentation while on-boarding new team members or passing the project off to someone else makes everything easier. Setting up a dev blog is 100% worth the minimal effort it takes.