How do Chinese restaurants 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.