How do they know

I remember when I was young, I never have to potty train my cats. They go outside themselves and bury their poopy in the soil then put soil over it after. So how did they learn this? All of them were rescued cats from the streets by the way.

 

It's instinct, don't ask me how come most dogs are not like that (but some are, some know by instinct they shouldn't poo near where they sleep - my frda learnt from day one not to poo in my room because of that, but not all the dogs are so smart). So basically it's instinct.

 

Thy inside cats have a litter box rather than a newspaper on the floor. I believe it is instinct, like Trellum mentioned. By burying it, it prevents predators from seeing and smelling it. If they predators don't know he's there, he will be a lot more protected.