After a device is set up and running

After you set up your device (say raspberrypi), and get your videos playing, do you still need internet?
I have a mobile application where I will be playing the same videos and there is guarantee of internet availability.

At least a bit. info-beamer hosted is a “mostly online” system. Most content you install will play just fine if the device is temporarily offline. But you need at least some connectivity every now and then as the device will otherwise reboot after a few hours and stop completely after a few days. There is no full “offline mode” yet.