I am still figuring it out since it is my hobby and I’m unable to devote much time to it. But I think it will be something like Ubuntu live disks which enabled you to try Ubuntu by running it from a DVD. You could run anything like web server, save files, settings etc. Only they would not persist after a reboot since every thing was saved in RAM. Only here it’ll be a write locked SD card instead of a DVD.
I’m also sure there must be a name for it and step by step tutorial somewhere. If only Google was not so bad these days…