Where is OAuth 2.0 typically used, and what do these uses have in common?

Social networks (login and sharing), cloud storage access, payment integration in online shops, access to external APIs, and remote control of IoT devices; in each case one service acts on a user's data held by another service without learning the user's password.

• Social networks (Google, Facebook, X): "Log in with..." and sharing content to your timeline from another app.
• Cloud storage: an editor or backup app reads and writes your files in Google Drive, Dropbox or OneDrive.
• Payment systems in online shops: the shop gets a limited authorisation from your payment provider account, not your login.
• External APIs: a developer's app fetches weather data, displays maps or reads calendar entries via another provider's API.
• IoT devices: an app or voice assistant gets remote access to control your thermostat or lights.

The common pattern is always the delegation triangle: the user owns data at service A, application B wants to use it, and B gets a scoped token from A's authorization server instead of A's password. Where the use is "login" (the first bullet), OpenID Connect is the standardised way to do it.

Go deeper:

• OAuth (Wikipedia): Uses — real-world deployments of OAuth across providers.
• Social login (Wikipedia) — how "Log in with..." works and its privacy trade-offs.