Most Shinigami receive a blank sword (Asauchi) to imprint upon. Ichigo’s forging is unique because he is the Asauchi.
The steam that clears to reveal the dual blades signifies the end of Ichigo's identity crisis. He no longer "uses" Zangetsu; he is Zangetsu. Looking Toward the End 13 : The Blade Is Me
For over a decade, Ichigo Kurosaki fought with a "false" Zangetsu—a blade born from a misunderstanding of his own soul. In the finale of Thousand-Year Blood War Part 1, titled that deception ends. This isn't just a power-up; it is a profound psychological reconciliation that redefines what it means to be a Soul Reaper. The Death of the "Old" Zangetsu Most Shinigami receive a blank sword (Asauchi) to