Using the APIs
TaskCluster uses a microservices architecture, with each service exposing a small set of API endpoints. Those endpoints, and other details of the services, are documented in the reference component.
All TaskCluster components use Hawk over SSL for authentication and authorize access based on a set of "scopes" associated with each client. Credentials and scopes are managed by the authentication component.
The details are documented here, but note that you can use one of the fine TaskCluster clients to handle all of these details for you. They are available in a variety of languages, and are kept up-to-date with the latest APIs.