If I update a podcast episode on my server, when does it get updated on Apple Podcasts?
Apple hosts a directory, Apple does not host the files of podcasts.
If you update the file of a podcast on your server, but do not indicate that you've updated it in your RSS feed, then only new downloaders will get the new file (obviously).  If you update your RSS feed to indicate that there is a new file, then many podcast catchers will figure out they need to re-download the file.  Others will simply list it as a new file again.