I use "Focus Plant: Pomodoro timer" on Android, but there is an iPhone version too. It's free, with ads, but there is a subscription that removes them. I can only speak to how it works on Android, but in addition to blocking other apps (or at least adding a barrier to get to them) it gamifies things by letting you earn "raindrops" for focusing. Those are used to water little plant monsters, which there are a ton of. You can set a specific timer, regular or pomodoro, or just a count up, and it will show you a little icon for other people who are also currently focusing, which is kind of nice. It's a bit silly, but the added layers of game and very light social aspect add a little that help me personally. It's very usable with ads if you want to try it without committing money. I personally subscribe because it makes some of the game bits (sort of loot crates) more fun, but that's definitely optional. (Good grief this sounds like I'm astroturfing for the company. I'm not, I just tried a bunch of other apps too, and this one has a combination of features that helped my dumpster fire of a brain.)