I’m pretty sure each person chooses a document/book to swear on that is core to them. So most people in the US would choose the Bible because they identify as Christian, but if a Jewish person or Muslim person was sworn in they could choose the Torah or Quran. And a non-religuous person could choose anything that they could convinceably argue is important/core to their values.

Disclaimer: I did no research right now to confirm this but that’s what I remember.

Specifically, the joke starts with someone asking you if you want to join their pen club. They proceed to explain that all the other members are numbered and you would be member #15, so they write pen15 on your hand and there ya go.

Interestingly the video seems to be loading fine for me through OP’s link.