Built-in overlays are overlays that are attached to a specific background. If a background has OL in the name, this means there's a built-in overlay, although some backgrounds with built-in overlays don't have the OL.
Built-in overlays are automatically placed at the highest layer, meaning your characters will automatically be behind the built-in overlay.
You can use the background's name as the built-in overlay's name to layer or move the built-in overlay. You have to add it with the background's name to layer, scale, or shift it. Check out my Overlay Guide to read up on placing overlays with the background name.
You must add the overlay with the background name. This will not work if you use the create command. Using the create command will only add a duplicate overlay to your scene.
If you want to completely remove the built-in overlay from the background, you will need to add the overlay to the background like above and use the overlay clear command.
You can use any built-in overlay with any background. Built-in overlays are exactly like regular overlays; the only difference is just using the background name as the overlay name.
If you want to use the desk overlay from INT. INTERN OFFICE OL - DAY and add it to your own background, you can!