Hello Karn,
Welcome to the MS Q&A platform.
If you come via Databricks partner connect and sign up for a partner product(ex: Fivetran), then the usage billing information is provided to the partner(as you sign up with Fivetran).
For the other question, the APIs would depend on the partner product integration and the partner's capabilities. I recommend contacting the partner directly for more information on the billing process and API they have in place for publishing the usage information to Databricks.
I hope this helps. Please let me know if you have any further questions from my end.