The paper mentioned in the video is here:
Founded in 1920, the NBER is a private, non-profit, non-partisan organization dedicated to conducting economic research and to disseminating research findings among academics, public policy makers, and business professionals.
www.nber.org
As far as I can see, it is all about the the effect of various interventions on the prevalence of mask-wearing. The reduction of Covid infections gets mentioned only in an Appendix to the paper, and I think it is in the context of the cost-benefit analysis of mask-wearing, and there is no clear statement of the effetivenes of mask-wearing in the actual study. The 11.9% reduction mentioned at 3:33 in the video simply does not exist in the paper - in addition to skim-reading, I searched the whole paper for "11.9" and it was not there.
You are quite right about reading the original paper, which does not at all support the video's message about the importance of mask-wearing. That is not to say that mask-wearing is not important - it's just that we need to look elsewhere for evidence.