it appears when browsing games available on ITCH  that there are some iPhone games.

• if someone buys one, how do they install in onto their iPhone ?
• does it only 'install' on a phone that has been 'jailbroken' ?
• or is there some way to 'sideload' a game (that did not come from appstore) onto a normal iPhone?

I believe when a game is labelled as having an iOS version on here, it will either mean a build intended for use on Mac OS, or will have a link that leads to the appstore from the game page for phones to get the game from. As far as I'm aware, you can't sideload things onto an iPhone without jailbreaking it first.