It's a little more complicated - in AAD, the app developer has a single app registration, which is the app side of AAD. The app can ask for certain permissions, specify trusted sign-in URLs, manage client secrets, etc.

Then, on each AAD tenant that wants to use this app, an admin for that tenant would create their instance of the app, called an enterprise application or service principal. That second object can choose whether any user in that tenant can sign in to the app or not, and can assign AAD roles to users/groups.

It is very common for apps to have a single tenant, and therefor the app registration and enterprise application are both in the same tenant, but if the app registration would allow it, any tenant can create their instance, and assign users from their tenant any role they desire.

This means that it is possible to have an app managed really well within your tenant, yet make the mistake of allowing sign-ins from other tenants, and thereby outsiders could still have full access...