OAuth 2.0
OAuth 2.0 is an authorisation framework that lets an application access a user's data on another service without ever seeing the user's password. When you click "Sign in with Google" or allow an app to read your calendar, OAuth is usually what makes it work.
The user approves the request on the service's own login page, and the app receives an access token with limited permissions (scopes) and a limited lifetime. The authorization code flow with PKCE is recommended for web and mobile apps. OAuth handles authorisation; OpenID Connect (OIDC) adds a standard identity layer on top of it for sign-in.