Oppo
Allow App Installs from Unknown Sources auf Oppo
Coloros treats every app outside the Google Play Store as an unverified source. This is a security layer built into Android, and Oppo enforces it per app rather than with one global switch.
If you try to open an APK from your file manager, browser, or a third-party store without granting permission first, the installation simply fails or gets blocked with a warning screen.
Where the permission lives
The setting is not a single on/off switch for the whole phone. Each app that is allowed to install other apps, such as your file manager or a browser, needs its own permission entry.
Go to Tap on Settings ➔ Tap on Security ➔ Tap on Unknown source installations ➔ Turn On/Off the desired App.
Only the app you enable there, for example the file manager you use to open the APK, gets permission to trigger installations. Other apps stay blocked until you repeat the same steps for them.
When to leave it off
If you only install apps through the Play Store, leave every entry in this list turned off. Enabling it for an app you do not fully trust increases the risk of installing modified or unsafe APK files.
Turn the permission back off once the installation is done if you only needed it for a one-time APK install.