HTML, and all apps, in a default Virtualmin configuration, would be located in /home/domainname/public_html
This is pretty standard across many types of shared hosting and not unique to Virtualmin. You can access files via ssh, FTP (if enabled by your web server owner), the File Manager in Virtualmin, the Upload and Download module in Virtualmin, or the Edit Web Pages module in Virtualmin (which is a simple WYSIWYG HTML editor; note Edit Web Pages only exists in Virtualmin Professional at this time).
We also have a new file manager in development that includes a nice HTML5 text editor with syntax highlighting and such. It should be out in the next major version of Virtualmin in a few days. It will be included in both Virtualmin GPL and Virtualmin Professional.
Disclaimer: I work on Virtualmin.