Flatpak.
On the installation page you have a direct link to Flathub, but if you do not have Flatpak installed it is useless. It would be advisable to have an installation page from the repository and another from Flatpak with the instructions.
- Installation (Repository)
- Installation (Flatpak)
Each distribution has a particular way of installing. It is advisable to send the user directly to the official installation page. https://flatpak.org/setup/
The command line is obsolete.
flatpak install --user flathub org.libretro.RetroArch
Some time ago it is not necessary to put all that, so it works.
flatpak install retroarch -y
If in the future they plan to use the beta repository, then it is placed, to avoid confusion.
flatpak install flathub retroarch -y
(The ‘-y’ is to make life easier.)
Flatpak is about making it easy. Using directly from the software center (for that it is) or downloading the package directly from flathub would be more practical. For example: (After installing Flatpak) “Now you can search directly at your software center or download the Flathub installer from here.”
Ubuntu PPAs.
The ‘&&’ and the ‘apt-get’ are no longer used and Ubuntu recommends not using them anymore. I know how to use ‘;’ and ‘apt’. I don’t know the reason.
It is no longer necessary to update after installing a repository. It is updated automatically.
Installing “retroarch *” installs “fonts-mplus fonts-roboto fonts-roboto-hinted libretro-core-info retroarch retroarch-assets” none of this works, everything must be installed from the “Online Updater” even the cores. Also, when installing the testing repository, the terminal says that the cores are installed and updated automatically.
Almost all things are AUTOMATICALLY UPDATED when there are new changes in the Github, so you’ll always have the latest CORES, LIKE IN THE ONLINE UPDATER -> CORE UPDATER (which is disabled in this PPA).
Something like that:
sudo add-apt-repository ppa:libretro/stable -y; sudo apt install retroarch -y
sudo add-apt-repository ppa:libretro/testing -y; sudo apt install retroarch -y
And … the video has all the obsolete information. 
Regards.