Burgers at McDonald's are assembled to order - meaning the condiments and extras are added when you place your order - but the patties themselves are kept in what's called a UHC (Universal Holding Cabinet). That's where they stay warm until it's time to build your sandwich. The one exception is Quarter Pounders, which McDonald's policy requires to be cooked fresh every time you order one.
Ordering hacks at Taco Bell reveal ways to enjoy meals for less by creatively swapping menu items. For example, mimicking the Beefy 5-Layer Burrito is possible by modifying a Cheesy Bean and Rice Burrito for only $2.79 total, translating to a savings of $2.20 compared to the original.