They give birth to live baby snails, several dozen at a time, and can do so via parthenogenesis so all you need is one snail. Eventually you'll have hundreds.
You don't breed them they do it all by themselves, just put them in a planted tank or any tanks with a substrate and throw in some food and they do they do the rest.
depends on the snails if they are trumpet snails they will pick up and eat any left over food from the fish in the tank bottom and also possibly algae (but mostly left over food)