Personally, I went from Google Keep to Standard Notes to NextCloud Notes, but the process is similar to go from Standard Notes to Joplin.
There's instructions for how to transfer from Google Keep to Standard Notes here[1]. Then, export from Standard Notes via a "Decrypted" data archive[2], extract the plain text files from the resulting archive (I don't recall the exact path), rename them all from .txt to .md using zmv or a similar pattern-based file renaming program, and import them to Joplin as-is[3].
You'll lose formatting, unfortunately, and probably other things like attachments and checkboxes -- I'm not entirely sure, as I didn't use any of the more advanced features of Google Keep.
There's also instructions to go from Standard Notes to Joplin directly[4], bypassing plain text. That might preserve formatting and such.
The Darwin on ARM Project[1] attempted just that. It depends on what you mean by "iOS" if you want to say they accomplished it - for example, they got XNU, the kernel, running on a Nokia N900, but they never got a UI up and running (only single-user mode).