They're just trained to use litter trays.
My 3 cats never needed litter trays until they got old and started to become incontinent.
well, what did they do, otherwise? cats' instincts are to go somewhere where they can bury their mess, and that's why they use litter boxes. you don't have to train them, just show them where the litter box is. super convenient, compared to dogs. but also dogs are the bomb diggity