Unless you've jailbroken your device, the iPhone won't allow you to install apps from sources other than Apple's App Store. Apple is very careful about allowing third party software onto its devices, since this can expose users to malware and viruses. The problem is that there are times when you need to get an app on your phone or tablet that's not in the App Store, such as for a business or personal reason.
Fortunately, the iPhone can be configured to allow you to download apps from unknown sources with a few steps. To enable this feature, you'll need to go into the settings on your phone and scroll down to the 'General' section. From here, tap 'Profiles & Device Management' or 'Device Manager,' depending on your version of iOS. Here, you'll see a list of profiles that are currently on your device. Find the profile for the app that you want to install and then tap 'Trust.'
Then, you'll need to make sure that the app is downloaded from a trusted source, such as APKMirror. When you open it, you'll be prompted to enter your passcode or use Touch ID/Face ID. After that, you'll be able to use the app.
Keep in mind that this will allow you to install apps from any source, even those outside of the App Store, so be cautious. If you're unsure, or want to disable this feature, you can navigate back to the Restrictions section of the Settings menu and tap 'Allow Apps Downloaded From' to turn off this setting.