For beef, any good steaks really. Skirt, Sirlion, Rump or even fillet. Pork I think usually is loin, tenderloin, belly or shoulder

Most importantly google velveting … that is what is going to make the most difference

Often the cheapest lean meat they can find. Sirloin or bottom round for beef often loin for pork. The secret to stir fry is in how they treat/marinate the meat with cornstarch and/or baking soda (and sometimes also MSG) that puts the very thin coating on it that tenderizes, browns faster and help hold on to the sauce. It’s often called velveting.