think of model-driven app as your D365/CDS environment
navigate to make.powerapps.com --> Apps --> Portal Management (that's your model-driven app)
the documentation Step 4 asks to modify that form, to modify you need a solution first
go to make.powerapps.com --> Solutions --> create your own one if you don't already have one, add the entities/forms that you want the SP integration, and then add the subgrid
this video explain overall about the model-driven app components:
https://www.youtube.com/watch?v=02NWfHRYkeo
this video goes step by step on the SharePoint integration setup with the Portals: https://www.youtube.com/watch?v=C8weejmRlmQ
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