I use the Obsidian app https://obsidian.md to keep track of ideas. It takes a little while to set it up and understand how it works, however that is time well spent.
Once you have some notes in there, you can open Graph View and a ‘Mind Map’ like view of your interconnected notes appears. As I am a visual thinker, I find that this is helpful.
Everything is private, as the notes are stored on your device. No one else (including Obsidian) gets to read them, just you.
There are several thousand plugins available, for added functionality.
Obsidian is free to use, by the way.
See also: What is the Obsidian knowledge base app?
