They can't, and they don't. Firstly, if you look at most huge menus, they are really mainly composed of relatively few basic preparations that only differ in a few ingredients.For example, most budget-friendly 'Asian' (Thai, Vietnamese, Indian, Chinese, mainly) restaurants where I live offer three to ten basic preparations with four or five 'proteins' to chose from plus a handful of 'unique' dishes. They'll look something like the following:Tofu withRed curry, veggies and riceGreen curry, veggies and riceLemongrass and chili sauce, veggies and riceSweet and sour sauce, veggies and riceChicken withRed curry, veggies and riceGreen curry, veggies and riceLemongrass and chili sauce, veggies and riceSweet and sour sauce, veggies and riceBeef withRed curry, veggies and riceGreen curry, veggies and riceLemongrass and chili sauce, veggies and riceSweet and sour sauce, veggies and riceDuck withRed curry, veggies and riceGreen curry, veggies and riceLemongrass and chili sauce, veggies and riceSweet and sour sauce, veggies and riceSpecialities...............Those are 21 menu items, but only nine preparations, really. If you add in imitation (vegetarian) chicken, beef and duck, that's a total of 33 items from nine preparations... Add some starters, some soups and some desserts and you're well on your way to a hundred item menu.Secondly, there's a thing called 'mise en place', which is serious cooks' religion (yeah, we're a pretty godless crowd otherwise). It literally translates to 'putting in place' or 'set up'. It refers to the process of putting everything you will need to access during service in a set place, which is readily accessible and follows a logical order. The term is also used, by extension, for the stuff that is set up. And that stuff is prepared to the maximum extent possible. Nobody will (usually) start dicing onions in the middle of service. That will have been taken care of before, along with precooking anything that will take longer than a few minutes to cook and won't suffer too much in quality through the process to a point where it can be finished in a couple of minutes. Blanching vegetables is a classic example. Fonds, basic sauces, decorations etc. will also have been prepared in advance, meats and fishes been cleaned, dressed, portioned etc. pp.Prep work is often delegated to kitchen helpers, apprentices and greenhorn cooks and looked down upon as a menial task by (IMHO) conceited and incompetent 'chefs'. In reality, it's the heart, soul and backbone of a well functioning professional kitchen.UPDATE:For more information on how professional kitchens achieve speeds and volumes that baffle many hobby cooks, see Jaya Powell's excellent answer.