RE: Fighting against AI Music

Obsidian is free. You can pay for some extras, like publishing your notes in the form of a website.

I never used OneNote, but I think its organization is hierarchical. If you make a note, you need to already know where to file it. If something falls under both 'psychology' and 'mathematics', what do you do?

In Obsidian, you just link the note to any other notes you want (e.g. 'psychology' and 'mathematics'), which is more similar to how our brains work, hence "second brain". But of course you can do the hierarchical thing too if you want.

Here is an example of a website made using Obsidian (i.e. just this person's personal notes made public):

https://publish.obsidian.md/discretecs/Logic/Deduction+rule

You can see the mind map (interactive graph) on the right, shows how this specific note links to other ones.

All your notes are stored in markdown files, which makes your notes vault future-proof. I.e. you can migrate your notes easily.