“The six landed descent stages remain at their landing sites; their corresponding ascent stages crashed into the Moon following use. One ascent stage (Apollo 10's Snoopy) was discarded in a heliocentric orbit after its descent stage was discarded in lunar orbit.“

Elsewhere, I read that the ascent stages were crashed into the moon to provide impulses for the seismometers left on the moon. Snoopy is still in orbit around the sun. And the one from Apollo 13 is in the Tonga Trench. Two fascinating lists:

https://en.wikipedia.org/wiki/List_of_artificial_objects_in_...

https://en.wikipedia.org/wiki/List_of_artificial_objects_on_...

As for littering the surface of the moon, I was surprised to see in videos that in addition to the scientific equipment ( and golf balls) they left on the moon, there was a lot of other little pieces. In one of the videos on the rover, they literally remove the cover off something and just throw it aside on the ground.