Setup

It's recommended to install Sidequest (unofficial application store). This then allows you to install any Android application (if you have the APK package); this is especially useful for File Manager, which then allows wireless transfers to the Quest 3S.
To install Sidequest, follow the online documentation. You will need to activate developer mode on the Meta account and debug mode on the device.
On Gentoo, you will need to install dev-util/android-tools and probably add your user to the android group. You might need to start the ADB server (adb start-server command) at some point but I am not sure if this mandatory.

Controls

To come back to the "home", press the Meta button (short press, not long press). Applications that cannot be quit directly within the application can be stopped this way.
To center the screen, use a long press on the Meta button.

Unfortunately Meta adds a lot of crap software to the device, which you unfortunately cannot remove easily.
4XVR Video Player is a great video player: support for all formats, local and SMB playback, and various setups (black, theater). The standard version is non-free, but the free one is already good (not sure yet what are the limitations)
Play'a VR and Commedia Player are good free players (supporting SMB), unfortunately they don't play older container formats like AVI.