Unless I'm missing some cultural giant taboo about giving someone else your leftovers (I'm not exactly familiar with Chinese culture), I'm on board with Elsa having hacked the system. With her insane magic abilities I wouldn't put it past her.
What?
I'm Chinese. You goddamn savages in (whatever location you are in) eat each other's leftovers? What about the bacteria/viral contamination from the saliva?
A lot of leftover sharing is done in a family or relationship, in which case you're already part of one big germ pool. But as a sometime scavenger of friends' plates, it depends on the food:
Something like a sandwich, I would cut or tear off the bitten edge, and eat the clean rest.
Anything that's cut up on the plate, like steak, or comes in big distinct pieces like prawns, should be perfectly fine, unless they've been drooling over their plate.
Pasta could have a bit of residual contamination from the back of a fork pushing things around, but I don't really worry about it unless they're known to be sick.
Soup I would pass on.