Thai beef meatballs, how to get chewy texture and how corn flour and baking powder function in recipe?

Baking soda or baking powder? Powder will produce gas bubbles.

Both baking soda (but not baking powder) and corn starch are prevalent in chinese cooking and its derivatives elsewhere in Asia. I am reasonably sure that they are used even in home cooking.

Corn starch is used as a binder and texturing agent in minced or finely chopped meat. I have also seen it used in home cooking on sliced meat in stir frying for texture, mainly to capture the meat juices and allow that to become a thin coating of sauce adhered to the meat.

Baking soda is used as a meat tenderiser, more often in commercial cooking and less so in home cooking from what I have seen. Again, this is used commonly in sliced as well as minced meat.

As for meat balls (and fish balls/fish cakes too), the bouncy/chewy texture comes from kneading a meat paste (just ground beef if not fine enough) and also the way they are formed, effectively by extrusion - a pinch of meat in the palm of your hand, form a fist and squeeze it out through gap between your curled index finger and thumb. These are always cooked immediately, I suspect for hygiene because of excessive handling.

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addendum

Though I have never seen it used, I can see how double action baking powder could produce those little bubbles in the puffier fish balls that you sometimes observe. This would certainly justify keeping everything cold and a hot bath immediately after extrusion. Much if not all of the tenderising function of the soda inside baking powder would be lost however.

I do not know any more today. The only other idea is that some sausages have a very similar bouncy texture. Perhaps someone can chime in with that know how.

I read the key is to keep the meat cold and to put in the freezer to keep it that way then take it out before it freezes solid and you do this several times during the process right up to cooking.

Flour can be substituted for other flour of the same type (in this case flour of any starchy grain: wheat, corn, potato, etc). The same thing with starch (corn starch, potato starch, etc). Temperature controls what the fat does, and the texture of meatballs is the behavior of the fat. Movement: too much stirring in a too warm room, melts the fat and makes them dense. Be careful the temperatures of your meat more than anything else.