It's just note-taking in a single app, but instead of organizing your notes into folders, you link them. It's what Obsidian made popular. The name, because I use it for all facets of life, if I go to the doctor or dentist, I write it down, if I write an idea, or journal, it's all there. Tiago Forte coined the term initially. You find a note on my second brain about it, in case you want to know more (I also have related blog posts)
Nice, yeah, some call it a digital garden, some just a notes app. There are different views, hehe. Cool, would love to see what you're building, once it's ready.
I'd link it to you but HN gets angry when you do that :(
I was reading more about 2nd brain and it's very interesting. I'm going to write a blog post about it and tie it into my project.