ESPN+ service is accessed using the ESPN app.
The ESPN app shows content from both ESPN subscriptions and ESPN+ subscriptions. ESPN has different content from ESPN+ and vice versa - there is no overlap between the two subscriptions.
The app actually has two different login/credentials: one for ESPN, and one for ESPN+
To access ESPN subscription content, you need to login with your ESPN credentials.
To access ESPN+ subscription content, you need to login with your ESPN+ credentials.
(to access content from both services, you have to be logged into the app with both sets of credentials)
To check or otherwise login to your ESPN/ESPN+ accounts: Open the ESPN app/Settings/Account Information and Settings/Subscriptions
Additional resources that you might find helpful: