Combine AP flour and yeast in a mixing bowl. Mix in old dough in smaller pieces, salt and oil. Knead until the dough is soft and elastic. Cover with a plastic wrap and proof at room temperature for about 3-4 hours. Place the dough on a floured board and evenly sprinkle the baking soda over. Knead and shape the dough into a loaf. Divide the loaf into 12-14 pieces.
Dust each small dough with some flour and lightly press them flat. Fold the side of the dough up with left hand, meanwhile press the upper edge down with your right hand. Repeat this step until you get the shorter edge and slightly round bottom. Pinch them together between the thumb and the index finger of your right hand. Place them on the board and let rise covered until doubled. Repeat the step of folding and pressing.
Place the dough balls in a steamer and process the final proof for about 25 minutes. Use the flour during the whole shaping process. Each small dough needs about 15 extra grams of flour to finish the kneading. So that the texture of the steamed buns would appear thick and with clear layers. To obtain a chewy-textured bun, long proofing time at the final stage should always be avoided. Take away the plastic wrap and steam for 20 minutes over medium heat with a lid covered.