Zettlr vs Obsidian
Both Zettlr and Obsidian are Markdown plain-text editors that can be used to link together notes.
Zettlr
Zettlr is a general-purpose open-source note-taking app. Fast, simple, and versatile.
Obsidian
Obsidian is a lot more sophisticated in its note-linking abilities. You can easily build and navigate whole knowledge bases with Obsidian.
Fan of Markdown note-taking apps? 🤔
Maybe Paper is what you're looking for.