First, you need to connect to your AGOL site, then add() the CSV as an item to AGOL.
Then, you can create a Feature layer object using the layer's url, then append() to the feature layer. See https://developers.arcgis.com/python/api-reference/arcgis.features.toc.html#featurelayer
Note: Make sure the field mappings are correct to ensure the append works. If you have example names for the AGOL site and items, I can send a test script for you.