The following video is the second part of how to use Canvas apps Lookup fields with CDS (Common Data Service), this video covers how to use the combo box to display lookup fields and using CDS entities or collections to display the data, also how to save the data with PowerApps Patch function and Flow.
Hope this helps you understand the nuances of these type of fields in CRM and now in CDS and the differences on the Dynamics 365 Connector and the CDS connector Next time I will explain the difference between Status Fields and OptionSets, these are not intuitable to understand and follow.
Also, the difference between the Lookup field and Customer fields
THIS VIDEO HAS A BLOG POST!
https://abesaldana.com/powerapps-canvas-using-lookups-with-cds/
watch?v=6b9zlkNeQFQ