Is it possible to use the import feature to update existing listings and match them by way of their URLs? None of the other matching options work as some listings share the same title, or address, or description, etc.
Yes, it is possible to update existing listings. Please set the ID of the listing in the CSV file and choose Update existing only (if you want to update only existing listings) or Update existing or create new (if you want to update existing listings or add new listings) import mode. But matching is made by listing ID, not by URLs
When you say matching is made by listing ID, do you mean the post ID number?
The below is one of my listing URLs when I’m editing it in the admin panel. Can I create a column in a CSV file with the below six-digit number to update the existing listing?
Yes, you are right. Listing ID is the post ID
Unfortunately, there is no possibility to add a custom ID to import listings because the listing ID depends on their order in your database. But if it is needed a custom ID number to a listing then please try to add a custom listing text attribute in Listings/Attributes. Then it is possible to add a unique ID number for each listing as a custom attribute