Yes, Jupyter notebook has used CodeMirror for nearly a decade. The official Jupyter notebook has never used Monaco, but the Google version (colab) uses Monaco in order to very nicely leverage LSP support (similar to VS Code). There is preliminary work under way right now to switch from CodeMirror 5 to CodeMirror 6 in the official Jupyter project, but that's going to be significant work (see https://github.com/jupyterlab/jupyterlab/pull/11638).
It doesn't seem like there is much overlap between Replit and Jupyter notebooks right now, as far as I can tell...
Yes it can. One example is the Juno app in the Apple App Store, which provides a very mobile friendly interface to hosted Jupyter notebook services. The same developer also has a similar app that provides offline support. I’ve also put effort into making my implementation of Jupyter in Cocalc somewhat mobile friendly…
It doesn't seem like there is much overlap between Replit and Jupyter notebooks right now, as far as I can tell...