I am trying to list all the available properties of an item, that comes from a collection of user items in AGOL. So far all the examples show 3 properties (title, type, owner), but there are more. How do I get to see all the properties?
For a give Item, you can access its properties with simple dot notation, Item.property, and so on. The actual properties for any item will depend on what type of content it is, but the possible properties are listed here: https://developers.arcgis.com/python/api-reference/arcgis.gis.toc.html#item
If you just want to list everything, try using the advanced_search with the parameter as_dict=True.
gis.content.advanced_search('your query here', as_dict=True)['results']
This will likely give you more information than you want, but you can always winnow down from there.