I believe getBody() is the raw body (a string, in other words). If you want a PHP object, you should use getResult() instead, which in your above case, would actually return a CreateCustomerResponse object.
No worries at all. Once you click on one of the APIs (like Customer), you will be directed to a new doc page with all of the endpoints available. If you go to a specific endpoint, like CreateCustomer, you will see that the Parameters takes a CreateCustomerRequest and the response is a CreateCustomerResponse. You can click either of these objects to be taken to their details page which shows all of the fields and functions to access or set the fields.
Note, you can also just click the doc link in the root Github repo and navigate from there. On the next page, you’ll see apis and models, so if you click models, every model that exists will be there. (like create-customer-request.md)