Register a new subscription

Nexi(Italy) told me to implement a billing service subscription plan, following these link
I’m trying to understand how to implement the subscription on my application, following the guide i see that i need to getSubcriptions API at first, after that getPlans, and for the subscriptions i need to call the getBillingIntent method, which registers a new subscription, this is correct?, or i need to do something else to register a new one?