@AndreyT1: Data Pages define how data is sourced from a system of record. By default, you will find one for retrieving a single record, another one for multiple records, and a (stubbed out) one for updating a record back to the source system. You can also mix-and-match - for example retrieve data from a RESTful service, but update a record in a database.
You can replace them directly in App Studio. Click on your data object, data pages, and then on the ellipsis on the far right next to the data page you want to change.