Only a small handful will be successful, and almost everything will still be available on the Apple App store.
I have 7 gaming app stores on my PC, plus a couple of one-offs that weren't initially in an app store, like Minecraft
- Steam - General all around best store.
- Epic - Offers a free game or two each week. Has the best discounts during sales, subsidized by Epic.
- GOG - Offers older games that have been updated to work on modern computers. Now offers modern games as well. DRM-free: you can download installers and use them offline without using a gaming client.
- Humble - Offers bundles of games at a discount. I don't use them as much because they abandoned initial features like always being DRM free.
- itch.io - Mostly smaller independent games. They've had a few massive bundles of games where all revenue when to charities. I also occasionally play free games on their site through the web browser.
- Xbox / Game Pass - Subscription service.
- Amazon Games - Free games for Prime Subscribers. You can keep the games even after your Prime subscription expires.
I understand, many people wouldn't want to deal with all those stores, but the vast majority of games I play from the other stores are also available on Steam. Steam alone has a bigger and better variety of games than any other game platform, including closed ones like on the iPhone or consoles.
All of these stores/platforms have public APIs, and there are a couple of apps (Playnite, GOG Galaxy) that can show you all the games you own across all the major platforms.