Flask user authentication
I would suggest using the flask-login extension, it makes session management really easy to add to your flask application, and provides a nice documentation which covers in details every aspect of the extension.
I don't think that flask has any authentication built-in, only support for tracking sessions.
Here are some snippets for basic HTTP authentication and authentication with some third-party providers. Otherwise you will need to roll your own or use a framework that has this baked in (like Django)
Here is a discussion thread on this topic with a useful link
Flask-Login doesn't, technically, do authentication - it does session management, leaving the (tricky to securely implement) authentication details to you. Something like Flask-Security actually implements both session management and authentication (also nice-to-haves like password recovery/reset and the like), at the cost of having to have explicit support for your database.