I had a friend locked that was once locked upstate and he'd usually on occasion eat things like Instant noodles, chips, cookies, candy, Slim Jim's, canned meats like Vienna sausages, mackerel, chicken, tuna, canned octopus, etc also canned spaghetti/ravioli, rice, and dry beans...
To “cook” the rice or beans you'd have to use tap water from the sink inside your cell and let it sit there for an hour or so. Now if you're a kitchen worker in prison sometimes they'll allow an inmate to use the appliances while supervised.
As far as prison meals go it's not the horror that some of these YouTubers make it out to be. Yeah, there's a lot of bad meals but also good meals depending on where you're locked up, especially if you're an inmate in prison with a health condition like my friend, for instance, was a diabetic (diabetes, high blood pressure, etc) so the meals would be a little “better” compared to the average healthy inmate.