When you create an A and AAAA records that point to MiaB, navigate to the ‘(TLS) SSL Certificates’ page to see a new option to get a Let’s Encrypt certificate for the domain and configured subdomains. At least that is how it worked the last time I used it.
The index.html file should be placed in /home/user-data/www/example.com/. MiaB will automatically serve this file for configured subdomains, so /home/user-data/www/example.com/index.html will be served for example.com and www.example.com when they are configured to be served locally. However, if you created /home/user-data/www/picstorage.example.com/index.html, this file would be served for picstorage.example.com, this way you can have subdomains serving different files.