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I looked in the search function but didn’t see anything about this, but as a new guy I see a lot of videos where folks are able to stick cards on their thumb vertically and I wondered if there is some sort of adhesive being used to keep the card from falling off. If so, I imagine it would have to be fairly weak (like the adhesive used on Post It notes) and not adhere to the card itself... or it could just be that since I’m unable to get cards to stick like that I’m merely looking for an explanation that doesn’t involve me having to admit that I’m just not good enough at this yet to do that...

Answer is no. The moves are possible solely due to moist and pressure. You don't need extra adhesives.
Problem is some guys have sticky fingers and some don't. For example I struggle with easy moves like Angel. I have to breath on my thumb most of the time to be able to do it. I never looked into more advanced "sticky moves" for that reason.

That’s good to know... I actually live in the Southeast where it’s hot, humid and very sticky, especially in the summer and I still can’t get the cards to hang on for more than a split second... of course, I’ve only been doing this for a couple of weeks (lol!!!) now so I have much, much, MUCH(!!!) to learn...

Another weird question... is it just me or are thumb propulsions really hard on a deck?? I’ve been trying to get the technique down and it’s taking a toll on my Bikes for sure...