What are in the linux board support package?

A Linux Board Support Package can be downloaded from the Qualcomm Developer Network, but what are the things in it? There seems to be no document at all.

I found some proprietary firmware inside. How to use it? And is it built as part of the default OS?

You don’t need it. All of the relevant firmware is available through typical firmware sources (linux-firmware), and all relevant pieces are already in builds.