In general, you should install apps outside of your phone's app store only if you have a very good reason to.

Sometimes, not every Android app will be available on the Google Play Store in your region, which means you'll need to install the APK file instead.

Again, you'll need to allow another app (this time Epic Games), to install apps on your phone. Once it's installed, launch Fortnite, sign into your Epic Games account and start playing.

There are multiple ways to install Fortnite on Android and iOS devices. Come learn how it can be done! It can get tricky.