I used to work on the Substack editor. It's built on top on ProseMirror and Tiptap (an abstraction on top of ProseMirror) last I checked.

There's a variety of other implementations out there to check out as well: - https://quilljs.com/ - https://lexical.dev/ - https://www.slatejs.org/ - https://draftjs.org/

If you're talking about liking the full experience with settings and previews, that I'm afraid is all custom built. I can't imagine an open source reusable one being out there, but I could be wrong!