How do I see a list of all my resources in Civ 6?
You can see a list of all your luxury, strategic and bonus resources by clicking on "Reports" (1) in the top left bar and choosing the "resources" tab (2).
This will show how many resources you currently have available. (3)
I have not noticed a way to see in which city your luxuary resources are currently used. But your understanding is correct. Each available luxuary creates 1 amenity in the 4 lowest-happiness cities.