C: You can create a custom connector using a OpenAPI definition file or a URL to OpenAPI definition.
B: On the Security page you get to choose how to authenticate to the API.

A: If you were to create a Custom Connector from scratch, then you whould have to study the API you have chosen and type in the URL manually here.

Reference:
https://carinaclaesson.com/2019/09/06/setting-up-a-custom-connector-from-an-openapi-file-and-utilizing-it-in-powerapps-and-flow/