The latter two used without the former are effective because, by this point in training, the dog has learned to associate them with receiving food, much like Pavlov's dog, who was conditioned to salivate at the sound of a ringing bell.
(the dog; has learned; to associate them with receiving food; T:by this point in training) Context(the dog has learned,List([89, 108))):(the dog; has learned to associate; them; with receiving food)
the dog has learned,List([89, 108))
the dog the dog
has learned has learned to associate
to associate them with receiving food T:by this point in training
them with receiving food