I eat at a lot of Chinese restaurants now and also while growing up. I often wondered how is it that the chicken, pork, and beef in dishes are always so tender. I can never replicate it when I cook. What do the Chinese chefs use to tenderize their meat?
One technique, but not the only, is velveting. Here the meat is tenderized in an egg-white/cornstarch mixture for 20+ minutes, then cooked briefly (a minute) in oil or simmering water with a small amount of oil prior to using in stir fries.
I've never velveted in straight oil but water/oil definitely gives the chicken that smoothness that Chinese Restaurants obtain and the shorter stir fry cooking time makes it much more tender.
Super thin slicing (you'll need to cut the meat semi-frozen to get such thin slices), plus cutting across the grain also lead to tenderness.
Baking soda (Sodium bicarbonate).
If you find the meat has a spongy texture aside from being very tender, then very likely the restaurant put baking soda (Sodium bicarbonate) in the marinade. The sodium in baking soda chemically reacts with the meat and make the meat very tender and soft.
Below is an except from the cooking section in Sodium bicarbonate (Wikipedia):
Sodium bicarbonate was sometimes used in cooking vegetables, to make them softer, although this has gone out of fashion, as most people now prefer firmer vegetables that contain more nutrients. However, it is still used in Asian cuisine to tenderise meats. Baking soda may react with acids in food, including Vitamin C (L-ascorbic acid). It is also used in breadings such as for fried foods to enhance crispness.
Personally I found the meat too soft and would prefer if they can just marinate in oyster sauce or soy sauce with some oil - acids also has a tenderizing effect on meat, although not to the extend of baking soda.
I believe it is their suppliers rather than their techniques which are decisive. You may not want to know what 'restaurant quality' pre-portioned meat-units look like.
In order to be tender and juicy, a great deal of added water is bound into the product with dubious adulterants -ah, additives. This is all generally legal and safe.
How the animal is raised makes a big difference: here in Beijing hormones are routinely added to pig-feed to fatten up an animal quickly without it over-developing muscles.
So, if you really want to know why their meat is so tender, ask for the names of their suppliers or, better yet, some of the packaging to investigate for yourself.
Water and corn starch slurry.
I talked to some of my Chinese food experts...i.e. moms. They actually suggest using a water and corn starch slurry. They would add this slurry to the meat, whether it is chicken, beef, or pork, and let it sit for a little while before cooking.
(One downside of this potentially is that it may thicken the sauce or liquid that your dish contains as this slurry is also used to thicken sauces and gravies.)
Marinate the meat with pureed fresh ginger in your marinade. It will make it fall apart if allowed to sit for a few hours in the fridge. Corn starch slurries are just so you get that gloss and thickening to the sauce when you are doing the stirfry. Don't know of anyone that uses baking soda though I wouldn't rule it out.