SQL Server, Sharepoint, Salesforce and CDS are managed connectors having 'tabular' capabilities. This means these connectors light-up aditional PowerApps tooling at design-time for named entities such as 'choose table' pane when adding the datasource and the abilitiy to use as the datasource of a form control. There is also additional abstraction which enables the tooling to map CRUD operations for entities.
None for this is documented so developers cannot presently add 'tabular' capabilities to Custom API connectors.
We need the ability and guidance to create Custom API connectors having the additional abstraction needed to make PowerApps understand that the API has 'tabular' capabilities.