They are not.
Back end is the data access and network layer. How you store data, how you represent data, what business logic and process do you do to the data, how do you implement security, authorization, authentication. Everything necessary that users don’t need to see that happens in your application.
Being well versed in both aspect makes you a fullstack developer.
A front-end developer can make a single page application given an API, but a full-stack can make the SPA, but also write the API that represents the data, decide on a strategy to balance the load on the service, as well as design the database that efficiently stores the data and allow business operations