For personal projects - A physical notebook, I'm an oldy too 👴
To really answer your question, I use
Notella : For short form notes like lists and quick thoughts - because the best notes app is the app you make yourself 😋
Notion For long form planning with a lot of moving parts - you can nest different form of content. Put a page inside a table inside a page

For dev notes, I use GitHub! - It's near the source + shared with your team
(I tend to get carried away and write essays in the pull request description ) (Works better if the repo is public, because you don't lose it when you switch jobs)
Blog/Youtube - For notes where I need to research a little, I write a blog post or create a video!
It's great because by writing, you really clarify the thought in your head + you get to share it with others.
Bonus: It's a great resource for your future self (I can never remember how to debug CI builds, so I just watch my own content 🙈)