Notes are quite great. they let your ideas free, help organise thinking, or remember things.
I use Obsidian. Use whatever works or whatever you like.
Email is a useful one - available on different devices and easy to make and save. Can have attachments.
Sync (between phone and computer) is useful, but not essential. Typing things twice takes less time than spending 60 hours a week researching the best and most secure self-hosted file sync solution and maintaining that for the rest of your life.