There will either be a remote or just buttons on the display itself. You can select the source of what's being displayed from a USB drive or SD card, that's how it's displaying the current image. Some of them have built-in casting options like chromecast.
If it doesn't have something built it, it will have HDMI in, which makes a chromecast, roku, firestick, or even just a PC a quick option.
Anyway to fix that to become a tv? I once bought a tv that had at one time been used for this purpose. Once it was unplug from the device storing the info it just became a flat tv.
my previous flatmate bought one of these digital menus used and it had a single DP input, he used a Chromecast and a box that extracted HDMI audio via TOSLINK and then a HDMI to DP converter for the panel. It worked great and it was a very cheap solution for the time.
I've seen Intel NUCs hanging from the backs of signage displays in Macca's when I lived in Melbourne. I guess pushing updates to the menus would be easier. My company used Raspberry Pis in our showrooms - admittedly it was implemented horribly. They all used SD cards which ended up failing due to write wear.
Interesting about the new models, would be keen to get my hands on one 😅