Xiaomi
Allow Apps from Unknown Sources auf Xiaomi
MIUI does not use one master switch for unknown sources anymore. Instead, every app that tries to install a third-party APK — a browser, a file manager, a cloud app — needs its own permission first.
This means downloading an APK with the Mi Browser and downloading the same file with a different app requires two separate approvals, even on the same phone.
To grant it, go to Settings ➔ Privacy protection ➔ Special permissions ➔ Install unknown apps. This screen lists every app that has requested the permission before.
Choose an app from that list, then use the toggle to Enable or disable Allow from this source, and confirm with Tap on OK.
If the app you need is missing from the list, trigger the install attempt once first — MIUI only shows apps that have actually asked for the permission.
Leave the toggle off for apps you do not fully trust. Every enabled source can install software outside Google Play review, so only allow apps you use to get APKs on purpose.