Where it fits
Auth0 provides a hosted identity service that an application integrates through redirects, SDKs and APIs. It handles the login service rather than running the rest of the application. Customer identity and workforce identity also have different requirements, even when both involve signing in.
How pricing works
Plans distinguish consumer and business use, monthly active users and selected features. Enterprise connections, machine-to-machine access and other add-ons can change the total. Count the authentication features the application needs, not just its registered-user database.
Choosing a setup
Auth0 is useful when login, federation and identity administration are substantial parts of the project. Clerk offers another hosted identity approach; Supabase and Firebase include authentication within broader backends. The application still owns its data-access decisions.
