"Apps from unknown sources" are apps that are not from the Google Play Store or the Samsung Galaxy Store, meaning that if the app is downloaded as an .apk file from a website, it cannot be installed on the Samsung device by default.

Before Android 9, there was a general option to allow or block apps from unknown sources. Now, under One UI, every single app must be allowed to install apps from unknown sources.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Apps
Step 3:  Open the  menu
Step 4:  Tap on  Special access
Step 5:  Tap on  Install unknown apps
Step 6:  Tap on desired  App
Step 7:  Enable or disable  Allow from this source
  1. Open the Settings
  2. Tap on Apps
  3. Open the menu
  4. Tap on Special access
  5. Tap on Install unknown apps
  6. Tap on desired App
  7. Enable or disable Allow from this source

Samsung Instructions
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