You can starting learning using the Sailpoint University trainings.
https://university.sailpoint.com/Saba/Web_spf/NA10P1PRD075/app/catalog/browse/categ000000000003340
The links are for the Admin and User level documentation for the NERM tenant for setup and use.
You will need to use APIs only if required to perform certain tasks that cannot be acocmplished using UI.
Yes, NERM is a separate license and you need to work with your Sailpoint PS team to get one setup. Since you are an ambassador I think you can request one for learning : https://developer.sailpoint.com/discuss/t/non-employee-risk-management-nerm-demo-tenants/62032
Hope this helps. Happy Learning!