How to Allow Installing Unknown Apps on Android
Android 8 and newer
Since Android 8, the permission is granted per app, not globally.
- Open the APK file from your browser or file manager.
- Android shows a prompt: tap Settings.
- Turn on Allow from this source.
- Go back and tap Install.
Turning it off again
Go to Settings → Apps → Special app access → Install unknown apps, choose the app and turn the switch off.