Joplin is an open source note taking tool. You can make notebooks, full of notes. It supports markdown, has a decent UI, supports a bunch of nifty markdown plugins, has search, and is just generally decent. Notably it has native (and good) apps for major desktop and mobile OSes. It also handles sync via your choice of clouds, lets you encrypt your data, and is open source. Also has browser plugins for capturing notes from a webpage.

There's a lot to like. True, like a lot of small open source projects, there are some rough edges, and progress is slow, but I believe they welcome new volunteers.

There are, however, a TON of note taking apps floating around at the moment. One is Roam. Roam is very expensive ($15/month; a premium price point in a space that's mostly free or a couple of dollars at most), and has a bit of a...well, almost cult-like atmosphere around it. People that like Roam seem to love it, for some reason. On the other hand, development is fast, and they support TONS of features. Backlinks seems to be a big one. Notably I think Roam is more focused around "blocks" of content than around freeform markdown.

Other options include Notion, Bear, Obsidian, Tiddlywiki, Evernote, Quiver, Dynalist, Workflowy, Standard Notes and many, many, many others. All are excellent options in this space. And now, Athens, which I suppose is trying to be a better, open source Roam?

Honestly, if you care a lot about note taking apps you should dig into it, there's a ton of options and interesting tradeoffs. And if you don't, then....eh, probably doesn't matter much.

Roam is https://roamresearch.com/