So what's stopping me, a random guy who doesn't work for uber eats, from walking in, pretending to scan a reciept, and walking out with free random food? What happens when the real driver shows up? Does McDonalds hold their line that they already made and packaged that order? Or does Ubereats insist they never picked it up?

Not a whole lot, actually. It happens on occasion, but less than you'd think - and in places where it happens frequently they hand out the orders rather than doing the shelf thing. If the person ordering has tipped well (yeah the drivers can see the tips) sometimes they'll get the restaurant to remake it and you'll actually get hot food, or some of the nicer delivery folks will let the restaurant know the order got snatched before unassigning from your order. And the restaurant has some incentive to remake your food because if you don't get your food they don't get paid - support is not good but it's usually pretty obvious when some kind of screw up has happened. The whole system relies on people being decent, and thankfully people are - at least poor people in cities generally are.

The local places are generally good at using a heat lamp, at least the places near me that allow you to order through their website, for pickup - which I'm usually picking up my own order because, like you said, those delivery apps are stupidly expensive even if you're ordering from somewhere actually worth ordering from.