The Importance of Public API Endpoints

In order to create my own form from scratch and do my whole progressive enhancement thing, I have to have access to that URL. And if not, if it's some sort of abstraction layer and these libraries are doing some sort of sort of a critical step in this HTTP back and forth, is this serialization? That captures all the data that needs to be sent and maybe how the function needs to be responded when it's sent to the server and then sent back or whatever. It's useful to make a distinction between I for lack of better terms, public and private API endpoints. If even if by public, I don't, you're still just using it within your organization, public