Developing a mobile app is a bit different than a website. Though, the UX/UI design part is pretty similar to Web Design, the difference in reflected in development process.
There are various reason why you may want to choose and app over website: It allows you to build apps with native features like camera (e.g. Snapchat), microphone (e.g. Shazam), location (e.g. Uber). Likewise an app, unlike websites, can directly use your phone's memory, graphics and storage, which allows for the development of very intensive games and application.
a. For Android Only
b. For IOS (Apple) Only
c. Hybrid App
For Android Only
We can develop apps specifically for android by using programming language Java or Google's own Kotlin, but the apps won't work in IOS.
For IOS (Apple) Only
Here we can use swift (Objective - C) to develop IOS apps, but, again, it wouldn't work on android
Recently with the development of Facebook's React Native and Google's Flutter, we can develop an app that works great on both android and IOS, sure we'll have to do some platform specific optimization, but its way easy than developing separately for both platforms. And since the world most popular app Instagram was built on React Native, there is no need to worry about performance issues