General questions about the API

I apologize if there’s an FAQ somewhere that I looked over. Been curious about these questions for a bit now.

What can the Halo API be used for? Is it for websites like Halo Tracker?

I was under the impression much of the code for Halo was not public. What is made public in this case and is it a “full” version of what is used at 343i? Hope that’s the proper terminology.

Could someone theoretically make a Waypoint clone with the tools available to them? Not suggesting anyone do so because Waypoint’s already solid and the official website.