How do you automate billing and eSIM provisioning for an MVNO?

I'm researching different approaches for building an MVNO platform.

One challenge I'm running into is avoiding multiple disconnected systems for:

Do most teams build these components themselves, or do they use a unified platform?

I'm interested in hearing about the architecture you've used and any lessons learned while integrating carrier APIs and billing systems.

Any recommendations or implementation experiences would be appreciated.