So you need to integrate our backend SDK into your API layer. That SDK will talk to the supertokens instance (that you have created after sign up). Together, they will create sessions and pass them to the frontend as cookies.
On the frontend, you will need our frontend SDK (As you already know). This SDK will do automatic refreshing of sessions + allow you to check if a session exists or get the userId of the logged in user etc..
Have a look at this 8 min video tutorial: