There are tooltips where help files exist. There was a time when it covered pretty much every Virtualmin option, but it’s been a few years since anyone was able to spend the time needed to keep it up to date.
Volunteers are always welcome. The tooltips are in a simplified HTML-based template language (covered here: Module Development - Webmin Documentation). You can find the existing tooltips and other online help here: virtualmin-gpl/help at master · virtualmin/virtualmin-gpl · GitHub
Adding a tooltip is super easy. You just have to create a file with the same name as the item label (or input id or name for the field in Authentic Theme). You don’t need to look at the source, you can just use a browser developer tools selector thingy to find out the ID for a field. Once you have the ID you just create a file named $ID.html (obviously replacing $ID with the ID you found in the form).
That said, I’m poking around right now, and it seems like we’ve still got pretty good help coverage. All of the core Virtualmin pages have tooltips for nearly all items. If you’re finding a specifically confusing item that doesn’t have a tooltip, feel free to make a PR to add it, or create an issue at github to request somebody else add it. (And, if it’s confusing, it might be that we need to improve the UI rather than add more docs, or maybe a little of both.)
Webmin is a much larger project and doesn’t have as much hand-holding, but you’ll still find some of the better loved modules have either pretty good online help (top left corner 
question mark) or tooltips or both. Postfix is a good example with excellent help and tooltip coverage (mail is so confusing for so many people we put a ton of work into making sure people could find help it they were looking).