How do restaurants keep their food warm and fresh for service?

good question fidaa

in most  fast food restaurants we must have an equipments designed for that target

but sure it depend on kinds of  itemes & holding times too

generally & overoll

holding capain, heat lamp , sandwichs transphair & binmarrine

we should have also microwaves too.

all of them such a necessities equipments for keeping the cooked itemes  fresh & warm too

un cooked itemes we must put them in freezing rooms or melting(fresh) rooms.

There's more than one way to skin that hog, but here's how I do it.I serve some very wonderful braised pork ribs in a Bourbon glaze that I make the day before. It braises for the prescribed time, then goes right into a bag, is vacuum sealed, and quickly brought down to35 degrees where it stays until the next service. When someone orders the ribs, I bring the product back to temperature in a rapidly boiling water bath, give it five minutes to rest and distribute, then cut and add some more hot glaze."Reheating" has a bad rep. Not because it's bad, far from it (the ribs actually benefit from cold resting) but because people want to feel like someone in the kitchen makes everything for them to order when they order it. I'd love to do that, but no one has9 hours between order and service :).Chefs do this all the time. It's called holding, and we make that roast or those green beans more or less the same way. That pasta? Was cooked within an inch of its doneness this morning and is just finished when you order.

freeze them when it is needed or keep them warm either by cooking or heating in a microwave

In fine dining and casual restaurant the prepared  uncooked, fresh items is put inside of the chiller and cook it instantly  if needed to make it fresh and hot in the table of the guest. But in baguette they cook volume based on their forecast quantity they needs binmarin or warmer at required temperature to hold the freshness of food and to remain it  hot. But as rule of food safety to avoid food borne case  it would be last4 hrs in warmer then discard.

it must be place in the food warmer all the time with a very low heat temperature to maintine the quality of the food you are going to serve for the costumer....