You can use a Select to get all the unique primary key values from the Excel table. You will need to insert the Excel values output into the From input, switch the right-side Map toggle to a single input, & insert in that single Map input something like
item()?['InsertPrimaryKeyColumnName']
Then follow that with a Filter array on the JSON of the CSV data, use in the condition…
Select Body Output does not contain CSV unique primary key value, like
item()?['InsertCSVPrimarKeyColumnName']
That will filter everything down to the rows in the CSV not found in the Excel table so you can then add them to the Excel table.
