Hi, I have an Unbraco site that has some API controllers, I've been told they want to run those APIs in their own app service that wont be public facing but pointing at the same database, the most logical approach would be to make a new Umbraco app that just contains these API controllers, I just wondered if it would be possible to create an Umbraco app that's just purely API without the back office/deployed site?

If you connect two Umbraco instances to the same database, I'm pretty sure you need to use Umbraco load balancing. Even if you somehow manage to make a readonly Umbraco installation without backoffice or the frontend, you still need the publishing/scheduling/subscription system to work. Why not just call the existing API controllers from the "own app service"? This is what we do with headless implementations.