Epicor P21 to Website API Documentation

Hi there! I am helping a client create a bridge from their eComm site (done in PHP/ExpressionEngine) to their P21 implementation, to help with managing inventory and shipping and all of that. I have been looking for composer packages or some sort of code that gives examples of this, but can’t find any. I also can’t find any documentation re: the P21 API for authentication or endpoints or anything.

Where is the best place to start with all of this? Or does anyone have a basic P21 API implementation that they could share (regardless of language)?