How is the radian of ancient traditional Japanese Dao made? Is it completely achieved during forging, or is it achieved by uneven soil covering?

I don't know much about smelting technology, but I have seen a Japanese documentary about the casting technology of Japanese knives. I clearly remember that there is a "most important procedure" in it, which is quenching. Because Japanese knives are not made of a whole piece of steel, but two or more pieces of steel with different textures are combined together, and they are closely combined through forging, and some become the blade and some become the back of the knife. Before quenching, the whole tool blank is straight, but when quenching, the deformation state of the blade and the back of the tool is different, which leads to bending.

For example, I wonder if you have eaten sweet and sour mandarin fish? In this dish, the pot is not cooked yet, and the mandarin fish, fish skin and fish are connected together; After cooking, the fish skin shrinks more than fish, so the fish becomes a hedgehog. Analogous to Japanese Dao, fish is equivalent to the blade, and fish skin is equivalent to the back of the knife.

It is said that this is actually a poor performance of Japanese Dao technology, because Japanese Dao imitates tangdao, while tangdao made by the same technology is straight-edged, and Japanese craftsmen can't overcome the problem of curved blade.