her belly grows bigger and her nipples pop out
NO
If the female is trying to get pregnant with a male dog then it will go on the males back but if the female dog is going on the males back while pregnant then it may means that they wanted to have another baby.
You don't need to tell anyone if a dog is pregnant, but if you want to, you can tell the dog's owners.
By mating with a male dog.
No
Sometimes.
doggy style. How did you think they do it?
Yes.
Often, a male dog is protective of the pregnant female dog, usually if he is the "mate." Sometimes dogs, mates of the pregnant dog or not, will act aggressivley towards the female dog, also known as the "bitch."
She could die.
If the male has not been neutered and/or the female has not been spayed, then yes. And unneutered dog can get any unspayed female dog pregnant.
Once the ovaries and uterus have been removed from a female dog, it is impossible for her to get pregnant. If a bitch 'believed' to have been spayed does get pregnant, it must then become obvious that the dog was never spayed.