Notebooks

Plain Text Format

Also see: personal knowledge management and LaTeX.

If you're putting effort into writing your own notes, consider not locking them in a proprietary format like Word or Google Doc. Instead, use plain text format such as markdown or LaTeX.
See Why Plain Text Matters and The Unreasonable Effectiveness Of Plain Text (business perspective)
Software: I recommend Obsidian: it's free and has a large community of users.