You can’t manage your own environment on the server module like you might with Conda on your own server. You have access to the packages and version listed on the page you found.
If you need a package that’s not currently available you can request that it be installed.
If you are making a “client” app that you would sell access to that’s great. Anvil has a users module and supports payments via Stripe (or you can implement your own via another API). The limitation on the plans are for compute and developer accounts.