I'd recommend Obsidian. Free and perfect for making and organising markdown notes, plus heaps of community extensions. And out-of-the-box you can make "canvas" pages where you can draw diagrams.

I'm interested to know more about what you mean by this.

Kagi

Reminds me of the surprised Pikachu meme

iHole

Just looked this up, had no idea but it makes sense

Thanks for the laugh

GPT Unicorn is one that's been going on a while. There's a link to the talk on that website that's a pretty good watch too.