The World Of Skk Girls summary:

In a mysterious world devoid of humans, Katana, Saya, and Kodachi are three inseparable friends. Their days are filled with laughter and games in the eerie quiet. But beneath this calm, an obscure presence lurks in the shadows…?! From the creator of “Muto to Sato,” Akatsuka-sensei, comes a new, uncharted adventure—innocence meets mystery in a world unknown!