I believe Obsidian are using ProseMirror or CodeMirror, the logo is at the bottom of CodeMirror so I assume it is the latter. There was this forum post from one of the Obsidian team a few years back asking about it

https://discuss.codemirror.net/t/implementing-wysiwyg-markdo...

https://codemirror.net

https://prosemirror.net