Using iTunes, Apple Inc. officially provides software that allows you to install third-party applications on your iPhone. To do this without feeling like you're breaking the law, you can use an alternative app store such as AppValley, TestFlight, TopStore, or iTunes. These stores will provide access to apps that Apple doesn't officially recognize.
Aptoide is another great option for those who want to purchase apps. It offers faster updates than Google Play and access to geo-restricted Android apps. To pay for in-store purchases, Aptoide accepts AppCoins (APPC). If you get an error message saying “your device is not compatible with this version”, try clearing the Google Play Store cache and data, then restarting the Google Play Store and trying to install the app again.
You can also try restarting your Android device, connecting to a VPN located in the appropriate country, and then opening the Google Play app. If you have a profile on your device from a trusted source such as your employer, you can install it. However, since it's an application developed by your employer, you'll need to contact them to update it and make it compatible with iOS 10. Unofficial apps aren't allowed in the iOS app store, so Apple won't recognize the developer when you try to use one on your device. Once you enter your password, Touch ID or Face ID, your phone will allow you to install, open and use the app since Apple has already thoroughly examined everything available for download in the App Store.
If not, a message will appear calling out the developer of the Untrusted app and saying that it can't be used on your iPhone.