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

1. Access the Profile tab under your KM console.
2. Click 'Manage Control App' button on the upper right corner above the profiles list.
3. Click 'Add', select Android Platform and fill-up the Package Name and the Application Name for each of the desired app (You can easily get the package name of desired app if you use 3rd party package viewer app from Google Play Store).

Part 2: Setup policy

1. In Profile menu, select the profile that you want to deploy.
2. Click 'Modify Policy' button.
3. Go to Android Enterprise > Application > System App Activation Setting: Apply.
4. 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 customer devices.