The core API contains the endpoints used to configure a host, as well as the underlying OS abstractions that they are built on. Usually you'll use Intecture Proj (proj/) instead, which reexports the core API, though for some applications, the core API will suffice.
The core API is organised into a series of directories for each endpoint (e.g. command/, host/ etc.). Within each endpoint is a providers/ directory that houses the underlying abstractions.
For API usage, check out the examples.