They learn the same way we do. We are told that green is green and not purple. If we would be told otherwise, we'd live believing that green is in fact purple.
We are also told that some things are bigger then the other. But 'bigger' is just a word that we relate to something. Just as we would relate the word green to a green object.
Parrots have the ability to see the relation between words and objects or sounds.
It is even believed that if you teach a parrot like this one from the moment it is born to the end of his life (which is a very long time) that you can actually have conversations with them as you would have with for example the neighbour.
For example;
Person; "Nice weather, don't you think?"
*Parrot sees the sun (or doesn't) and responds accordingly*
etc
This is exactly how we use our brain. All we do is connect the dots
