As the title says, what are the risks, I assume people don’t host these servers for no reason. Is it just the ad revenue they get from the popups when trying to start the movie.
Could they be running exploits on here that would make it not safe at all to do so?
Using the default browser on an iOS device should be fairly safe, assuming that you accept all security updates as they become available. Outdated browsers, on the other hand, are a major security risk.
For legitimate free movie sites (Plex, etc.), ads are indeed the main source of income, along with paid services like the ability to rent current movies. If you are smart about it, they may not collect too much of your personal information.
For the rest, the business model relies on pop-up based scams, hijacking machines for botnets, and ransomware.