< sits down and looks amazed at what people start delving into >
I'm not too sure what I can / cannot say about how BTB works, so I may be a little coy, I do actually like my current employment status
One thing I will mention about the key terms is that the first occurrence of that term might not be the one that is underlined (if the device / app supports that functionality), I believe there is something server side, when the BTB data is generated that determines what will appear as a key term.
You are correct that the BTB data is downloaded when a book is download, or at least that's the case on iOS