When you configure your instance, you can add in external providers, such as google. The a user can link a single profile to multiple providers.
In the example /ui/settings screen, you can see this in action. Register a user then visit that screen, at the bottom you can see all the providers that you've configured as options for the end user to link.
The attached image is from my own instance where the user registered via email, then linked their google account, and they have the option to also link their facebook account.