By having them treated for parasites as soon as possible and having a preventative program in place.

They get tapeworms by ingesting fleas or flea eggs. By controlling fleas, you're not going to ever have tapeworms.

Yes, unless your indoor dog never goes outside, although I don't think any dogs are like that.

Intestinal parasites can be passed inside the womb of the female dog onto her puppies. In other words, they can be transmitted before they're even born, or in the course of being born, or puppies can also get them just from the environment. An adult dog can pick up parasites by, say, walking through a dog park, then licking their foot, and ingesting an egg. And then, they will have the parasites.