Apps that are not listed in the Google Play app store are called Unsigned android Apps.
If you need to install such an app for your Android device, you can still do so, but you’ll need to enable settings inside Android that enable you to install unsigned third-party apps. These settings are located inside the Android Settings app.
Open the Settings app on your Android device.
Tap the “Security” option in the Personal section.
Tap the check box next to Unknown Sources. This enables your device to install unsigned, third-party apps from sources other than the Google Play app store.
Open your app’s APK file to install it on your Android device. You can transfer the APK file to your device by connecting it via USB to your PC and transferring the file over. You can also email the APK file to yourself and open it as an attachment or download it directly from your Adnroid device browser.
If the app is uploaded on Google Play store, this procedure is not required.