How does Android check whether an app store is a known or unknown source?
Starting with Android 8.0 (Oreo), the process for installing apps from unknown sources changed. Instead of enabling installation from unknown sources system-wide, users must grant permission on a per-app basis. This is how Android determines if a source is trusted:
Trusted Source: Apps distributed via the Google Play Store or pre-installed system apps (e.g., Samsung Galaxy Store) are automatically trusted and do not require additional permissions for installing APKs.
Unknown Source: Any other app attempting to install APKs is treated as an unknown source unless the user explicitly permits it.
When your app store attempts to install an APK, Android checks if the "Install Unknown Apps" permission (REQUEST_INSTALL_PACKAGES) has been granted to your app. If it hasn’t, the system prompts the user with a dialog to enable this permission.