Depends on what type of app you're looking to build. If your app will need data from any other place other than your own phone, you will need to build a server component.
It's easier to understand if you treat your phone app like a browser. your browser runs on your computer, just like your phone app runs on your phone. everytime that your browser/app needs data that is not locally stored (your credentials to log in, any analytics to show you, data that other users have saved to display to you) you need to retrieve that data on the server. that is, you need to make a call to the server.
there is no independent server side with mobile apps. it's the same type of server side that a website would have.
however, you can have a mobile app that doesn't require a server side. think of a game, for example, that doesn't need to retrieve information about any other users's progress but your own. as you play the game, the app can store your data locally. because you don't need to retrieve any other data saved with any other user's phone, you don't need to go to the server. you can just use your own phone's local storage to save and retrieve your own data.
in summary, if you don't need to communicate data between different phones, you don't need server side and all you will need is a "mobile developer". if you need to communicate data you will need a "mobile developer" and a "back-end/server side" developer. people mean different things sometimes when they say "back-end developer". i like to get rid of that miscommunication by just saying 'server side'.
let me know if i should clarify anything.