peopleinside my recommendation is to not start by installing it on your VPS if you're unsure how things will go. You can start by installing Flarum locally in a (W|M|L)AMP install to get familiar with the process and ask any question here ?
What I think is important to get is that we don't use SSH to install anything fancy in the system, we just use Composer. Composer is just a tool that installs the PHP dependencies you need in your project according to a list of requirements and the system.
Technically you can run Composer on another system, generate the
vendor folder, then send it over to your server, no SSH needed. You just need to be careful to run Composer in the same setup (PHP version, PHP modules, operating system...) so it downloads the right dependencies for your final server. You can also manually specify the target system in
composer.json so you can run Composer in a different setup but get the dependencies compatible with your final server.
Apart from that there's really no difference from any other web application you might install on your server. SSH is not used to do any trickery outside of the Flarum folder, it's just used to run the Composer client. Once Composer has run and the
vendor folder exists, Flarum behaves like any php application. You can move the folder around or just delete it.
As for backups, it's very much like other software. The database needs to be backed up. Flarum also writes files to
storage and some to
assets like for avatars or attachments if using the Upload extension. You certainly don't need to backup the
vendor folder because it can easily be re-created based on
composer.lock (those files you should keep a copy of as well if you've installed extensions as they define which version of which extensions are installed)