Basic – level zero – why’s of it is like this. The best way of installing anything on a good (any?) Linux system is to install packages from this system’s repositories (repos for short).
Packages are programs and various parts of programs (for example, core libraries in one package, cli tools for the same program – in other, graphical UI part – in the third).
Repos are sort of collections of packages ready to be installed and used (with all the metadata needed for it to be convenient). (Almost) each Linux distribution has it’s own repos. Repos and packages in the repos are maintained by package maintainers from that distribution.
Installation/removal, updating, etc. of packages are usually done using package managers. dnf is a cli package manager for Fedora, Gnome Software is a sort of GUI package manager (not exactly, but let’s leave it at this so as to not overcomplicate things here).
It’s best to use packages made ready for you by package maintainers – because they are tested (more or less), easy to install and remove without breaking your system and without leaving litter all over it, etc. – it’s a topic in itself.
Now to NVidia drivers. They can’t be included in official Fedora repository because they are proprietary. There’s a good page with explanation about such legal stuff and Fedora’s packages, but I don;t have it handy, I’ll find it if you’re interested.
Quite a number of such packages – good, useful, needed by people – but can’t be included in the official repo – are included in PRMFusion repository. May of (or all) package maintainers for RPMFusion are Fedora’s maintainers, so while it’s officially third-party, it’s still a known trusted and good source of packages/software for Fedora.
So the commands we gave you did the following:
Added new repository (RPMFusion) to the list of repos your system knows of. Technically you needed to add two repos, separately for free software and non-free software.
Installed some packages (containing drivers) from the newly added repository.
@refi64 have already explained this part more technically.
I hope my explanation shed a bit of light on the matter. I personally find it easier to learn as I go – but that’s me, we all are different. There are plenty of information on the matter, but you can quickly become overwhelmed by it if you try to read it all at once. And I don’t have useful links handy.
Well Fedora’s quick docs are a useful place to see, maybe to start. Though I haven’t read them all and can’t personally vouch they’re error-free and up-to-date. As far as I know Fedora’s always in need of people’s help with improving documentation, quick docs as well.
Things we did are covered in Usage and customisation chapter, look through first five subjects there.
Asking here is good too )