How can I activate system apps (preloaded apps in the device) inside of the fully managed device or work profile area?

You can use the 'System app activation setting' in the Profile.

Part 1: Add the package name of the desired applications in the KM's control application list
- Access the Profile tab under your KM console.
- Click 'Manage Control App' button on the upper right corner above the profiles list.
- Click 'Add', select Android Platform and fill-up the Package Name and the Application Name for each of the desired

Part 2: Setup policy
- In Profile menu, select the profile that you want to deploy.
- Click 'Modify Policy' button.
- Go to Android Enterprise → Application → System App Activation Setting: Apply.
- Click "Add +" and find the applications that you've added on the Part 1.

With these steps, the System Apps should be added successfully on devices.