It's not the body of the boy we saw in first chapter(the mc that copied), the mc we see is a diff entity like the mc just copied the face and the shape of the body of the boy, you can even see in first chapter that the mc left the bones of the boy meaning that they did'nt switch soul or something. He even left a flower or something to the bones.
The mc is like a doppelganger.
And to copy the DNA.. Little Mushroom technically devoured the boy.. with consent, since the boy told Little Mushroom to eat.. to not waste the nutritious body.
The real soul of the body already dead and it was little mushroom in the body idk this manhua gives vibe sad and happy at the same time