"Where does the yield comes from?" If you're considering Fold, ask yourself "where do the rewards come from?"
They're selling your shopping data and giving you a 1% cut.
Discussion
For the gift cards portion, they buy them in bulk at a discount and give some of that as rewards. Same for any other gift card company like BtcCo, Bitrefill, etc.
Thought the exact same thing when I listened to that recent What Bitcoin Did episode. “So where does the bitcoin come from?”