In the Samsung secure folder you can add already installed apps or download new ones from the Play Store or Galaxy Store. apps that are installed directly into the secure folder are invisible in the system. means they are not listed in the app list in the settings. for apps that already exist on the Samsung device, a copy of the installation state is created and copied into the "secure folder". so it is a new installation.The installed apps can only be started if the password, PIN or pattern for the "Secure Folder" has been entered.

Android 10 (One UI 2.0)
Step 1:  Open the  Secure Folder
Step 2:  Tap on  Add apps
Step 3:  Select desired  Apps
Step 4:  Tap on  Add
  1. Open the Secure Folder
  2. Tap on Add apps
  3. Select desired Apps
  4. Tap on Add

Samsung Instructions