How do they Prepare Steak so fast in Restaurants?

When I worked the wheel at a local restaurant, we served 4oz medallions of filet mignon that went from fridge directly to grill. Only took about 5 minutes on each side, then the steak was plated and sent to the table. There was no "wait until room temp", oven or rest stage. The rest stage wasn't necessary because we weren't pre-slicing the steak before sending it out. By the time the steak leaves the grill and makes it to the table, it has hit a satisfactory rest period.

Larger cuts would require the oven, but those too, would not require waiting until room temperature nor the rest period.

Another question is how do they manage the steaks? Do they have several in freezer ready and when the stock goes down, they start to thaw the one that's frozen?

I can't speak for chain restaurants. The restaurant I worked in would order and prepare the fresh protein twice a week (Thursdays and Mondays). The protein was held in the walk-in cooler. When rush comes, the chef that works the wheel uses several 6" pans that separately contains all the proteins in a smaller fridge nearby.

Of course this is only my experience. Like I said, I can't speak for larger restaurants.

As far as I have experienced steaks are rarely brought to room temperature before cooking except for steaks cooked 'bleu' or 'blue'. You also have to consider that kitchen equipment deliver much more heat than your regular domestic stove. Furthermore the cooking equipment eg chargrills, stoves, flat grills, etc... are always on, hot and ready to cook. In addition, chefs are skilled and thus fast – in larger brigade, it is the more experienced chefs who operate the 'grill' section. Once the steak is cooked it has to be kept warm until the meals for the order the steak is part of are ready to go. Depending on how busy the kitchen is, the steak, particularly if cooked, rare to medium, will be taken away from the grill/stove/oven before a few minutes before it is fully cooked. It will finish cooking while resting. So skills, equipment and a good sense of timing all contribute to make cooking steaks in a restaurant kitchen much faster then in a domestic environment. As for 'managing' steaks that should be an other question.

One could also cook the steak "low temp" or "sous vide" to the desired doneness, then chill. In this case a high heat sear on a grill or flat top would only take a minute per side, to brown or form crust...greatly decreasing the time it takes from order to plate.