Obsidian, it's a really good Markdown editor

Yep, it's ObsidianMD, like the other person said. There's a ton of stuff there, such as being able to create tons of links between files and even links to files that don't yet exist, and then navigate those links and color code them in the graph view.

I also have RNote for handwritten notes, though that's more specific to Linux where Obsidian is cross platform