How to get the auth0 domain for local testing

I have been looking into your tutorial here which talks about protecting NestJs API with Auth0 access token.

At a certain time, they executed an auth0 apis create command using auth0 CLI which created the API domain and audience. I would like to know what that domain is and how can I configure one for my local testing. Also, I would like to understand what is the need for creating this domain as my need is to authorize my internal API call using the access token which I have in my front end after authentication