How can you write a computer program if you don't understand how every library you're using works down to the smallest detail? Humans are fairly good at working with building blocks that they don't fully understand.
In your analogy, there is me the programmer and the library's programmer. If the library has a consistent behavior, I need only to know how to use it, I don't need to know how it works internally. But the library's programmer needs to know how it works. He may not need to know how the building blocks, he's using in his library, work. but he does need to know how they interact to be able to deliver new features and bugfixes. (I think) He can't add features and correct bugs by trial and error (at least not all the time :)) .
So In your analogy, I think the researchers that came up with the BERT model are the library's programmer and not the programmer.