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.

Applications

Unfortunately Meta adds a lot of crap software to the device, which you unfortunately cannot remove easily.
Play'a VR and 4XVR Video Player are good free video players. Avoid Bigscreen, it's impossible to quit this application on the Quest 3S!