Virtualmin doesn’t have anything do with the repos. We’re always using the system package manager.
On yum/dnf-based distros, it is always in /etc/yum.repos.d, and our repos are configured in a package called virtualmin-release (which sets up the key and configuration).
Where things get tricky with new distros (even when those distros are allegedly compatible with RHEL/CentOS) is that setting up EPEL and/or SCL may be different on different repos. And, we may not detect the OS appropriately to choose the right virtualmin-release package or setup the right EPEL and SCL repos. All of that is done in install.sh and OS detection is in the os_detect function in slib.sh. And, if any packages are missing in the new distro, it may be a problem.