Advice I wish I could give myself. Don't eat out if you can avoid it and if you do go in person, never order in.
In secret someone who cares less about your health than anyone you know prepares hundreds of meals a day and if they don't come in because they are sick they lose money. If they don't try to hide mistakes it will cost them. If they don't save the company money by picking up food off the ground or using yesterdays soup as a base for today's soup they are doing a poor job. There are very few ways a customer can prove these mistakes unless they are visible upon receipt. Poor reviews hidden from the public is the only recourse.
A worker coming in sick can be a safety violation if they don't wear the appropriate protective equipment (generally gloves and a mask). Picking food of the ground is a safety violation that could get a restaurant shut down, and moreover would open the restaurant up to civil torts. A restaurant is not going to risk permanent closure to save a few cents on food that falls to the ground, especially not in places where they grade restaurants on food safety.
As for using leftovers: this will shock you, but a large portion of menu items in even fine establishments use leftover items (that were not served to customers). Soups, stews, curries, etc., generally involve perpetually renewed bases, where the previous days leftovers "seed" the new day's mix. The meats in pastas and other starch-heavy dishes are usually trimmings from entrees in which the meat is the star. Meatloaves in restaurants always use leftover meats from the day before. Nearly all breads in bakeries involve the reuse of the sourdough starter, and indeed the concept of sourdough itself is premised on the reuse of the the dough.
> using yesterdays soup as a base for today's soup
That's not necessarily a bad thing. There is such a thing as a perpetual stew [0] in which a stew is replenished with ingredients over months or even years.
In secret someone who cares less about your health than anyone you know prepares hundreds of meals a day and if they don't come in because they are sick they lose money. If they don't try to hide mistakes it will cost them. If they don't save the company money by picking up food off the ground or using yesterdays soup as a base for today's soup they are doing a poor job. There are very few ways a customer can prove these mistakes unless they are visible upon receipt. Poor reviews hidden from the public is the only recourse.