There are just two main types of smartphones around these days. Apple iPhones use an operating system called iOS, whereas all other phones that are currently sold use the Android operating system. Apps for iPhones are downloaded from the Apple Store, whereas those for Android phones are downloaded from Google Play Store. (There are actually other sites where Android apps can be downloaded from but they're generally far less trustworthy).
Some apps are made in two versions, with one for each operating system. Others though are limited to whichever one of the two systems that they've been designed for.
The only real exception to the foregoing is that phones made by Huawei use something akin to Android but the US government won't let Huawei have full access to the code for Android. So they have their own distinct operating system, with apps having to be downloaded from Huawei's own app store, which has far, far fewer apps available on it than either of the big players have on their stores.