They also have to be evaluated by a veterinarian and pass a physical as well. They are insured and bonded for liability as well. They would learn just like they would learn to fetch. Every time they respond correctly to a sign, they are rewarded. It doesn't take them long.
Therapy dogs are professional visiting friends.
They learn their manners, and they might just lie there beside you while you read a book or sing a song. A physical therapist will many times use them to help a person learn how to use different parts of their body that has been injured, like brushing a dog with the arm that is getting better, or throwing a ball for the dog. One little girl just liked to have Emma lie on her bed in the hospital and whisper in her ear. Emma loved it, she could have stayed there all night. Here is Emma giving a demonstration to an elementary class about pet care.