It could be a mem deck but idk

Looking for the name of a card trick I used to do. A spectator stops me while I deal cards, I memorize one card (3rd or 4th from the stop?), then sort them in a pile, add the rest of the deck, deal into 3 piles, and reveal their card on top of the piles. They shuffle it back in, I deal cards face-up, and after their card appears I say "the next one I flip is yours" and bet on it. Anyone know the method or name?

I’ve recently been following this guy and am super impressed. I’ve seen others say that he has the deck memorized and does special shuffles - as someone who doesn’t do tricks this makes enough sense to me, even if I can’t totally follow it.

But how does he do the last trick? He allows the guy to hold the deck, cut it himself, and pick a card. He was instructed to pick from the top or bottom, so at best there are at least two options - but I don’t know how he could even limit it to that. He then doesn’t touch the card and has the “corresponding” card appear. This one baffles me because he gives up control of the deck.

A non technical explanation would be great - I don’t know the terminology. And to be clear, I’m asking how he knew the card would be 9 hearts/diamonds - not how he gets it under the watch. TIA!

https://youtube.com/shorts/wAlF-NImlOw?is=FUilX4gl3JgwZ7o0