API

I'm working with a developer to build a very specific community. Is there a CB API, or do we need to rely on the Joomla API?
I'm thinking about integrating all CB profile info (including encrypted password) as well as GJ and Activity, etc.

I see (searching the posts) that there is a CB API, where would I find documentation on it?

A CB API to do what? Of course we've API as CB wouldn't function without it. Do you mean public facing RESTful API or GraphQL API? No, we've neither. You can build a RESTful API using CB Auto Actions though, but you'll need to be more specific for me to provide an auto action example.

Sorry. I'm asking through a 3rd party - this is not my area of expertise.
We're looking for REST API in order to build a separate App in the future.
We will want pretty much the whole user profile integrated, All fields.
I'm not sure yet about GJ Groups, but possibly them also.
I was asked this: "If you can figure out and create an example of how login can be done via POST request, and data retrieval via GET request, that would be helpful." 

It's possible to login a user and respond with their profile data using a POST, yes. The below is an example of that.

www.joomlapolis.com/forum/developer-members-support/245843-default-password-and-redirect#333857

So yes it's doable, but you're going to need to provide specifics. What is this for? A mobile app? From JavaScript or from server side? There's a lot that needs to be answered. If the API calls are never exposed to a client it's easy enough to just use a secret token and support supplying a user id to retrieve profile data or really any data you want.