Virtualmin isn’t your operating system. You don’t install anything “on” Virtualmin (well, except Virtualmin plugins).
And, of course. PostgreSQL is fully supported just like Mariadb or MySQL, and there is a module for Postgresql.
To install it, use your OS package manager. You haven’t told us your OS, so I can’t tell you the exact package names. But, you can search for the postgresql packages, either using the command line or in the Software Packages module. You certainly need the client and server packages, and you probably want the Perl DBD::Pg module (installed from your OS package manager, not from source from CPAN, never install from source when you can install a native package), as well, as it makes the Webmin module faster and more capable.
If you’re on a very new installation of Virtualmin (from the Virtualmin 8 repos, rather than the Virtualmin 7 repos), you have a minimal Webmin package, which will require you to install the webmin-postgresql module, as well.
Once installed, you can preconfigure PostgreSQL for use with Virtualmin by running:
virtualmin config-system -i PostgreSQL
You probably want to browse to Webmin->Servers->PostgreSQL and make sure the server is found and operational and can be interacted with. Since it hasn’t been part of the standard install. You may need to click “Refresh Modules” if you don’t see PostgreSQL in the Servers menu, as it may be in Un-used Modules.
As for NodeJS, you install and run it just like you would under any other circumstances. Virtualmin has nothing to do with it. Install it with your OS package manager, if you need a newer version, NodeSource is probably the best option for OS native style packages. You proxy to your nodejs apps, just like you would in a non-Virtualmin environment. You can generally just follow the documentation for whatever app you’re trying to run for your OS. If you want to use Virtualmin to create the Proxy rules, you can certainly do that, or you can add them manually according to the docs for your app(s), Virtualmin doesn’t mind either way. Whatever you find easiest.