Sift labels new Gmail by importance, on a server its owner runs. When mail arrives, it reads the new messages and applies a label to each one, guided by a rubric the owner keeps in their own private GitHub repository. It learns from how the owner handles their mail and from their edits to that rubric.
Sift is a personal tool. It runs only for the Google account of the person who set it up, with their own Google Cloud project and OAuth client.