That doesn’t make sense on a couple of fronts.
Virtualmin is a system service that needs root access to the system to do its job. It is not a “web app” in the sense of running under a web server. You don’t install Virtualmin “into a subdomain” because it doesn’t run in your web server…it manages your web server.
And, a subdomain is just a name. Virtualmin can listen on the IP that a subdomain name points to (by default, Virtualmin listens on all addresses, so if you have a subdomain name pointed at your Virtualmin server, Virtualmin will answer).
It also doesn’t make sense to install Virtualmin “on a directory”. Virtualmin, again, is a management tool. The dependency list includes thousands of files., and a dozen other system services. Virtualmin is not a web app, it’s a management tool, and it needs administrative access to all of the services it manages.
In short: Virtualmin doesn’t run under the web server.
The File Manager (with text editor) is a Webmin feature. It is not exclusive to Virtualmin. Maybe you just want Webmin? You can’t install it on a subdomain or in a directory either, but it’s a much simpler thing to install on an existing production system. It won’t disrupt anything and does not need a freshly installed OS.