The most simple one is that one end customer accesses the APIs of just one telco operator (single-operator relationship).
Big customers also have the ability to handle multi-operator relationships.
Smaller customers can therefore use the services of an aggregator. The aggregator can enable e.g. a single-sign-on for the end customer and handle the authentication and authorization towards the telco operators.
As alternative solution also a single operator “API roaming” is possible.