WARNING: The following contains spoilers for Jujutsu Kaisen, Chapter #147, by Gege Akutami, John Werry and Snir Aharon, available now in English from Viz Media.
Jujutsu Kaisen Chapter #147 recently revealed the secret of how Masamichi Yaga, principal of Tokyo Jujutsu High, created Panda and other Cursed Corpses. The revelation comes at the cost of Yaga's life as punishment for causing the Shibuya Incident and luring Gojo and Geto into fighting each other. Yaga reveals his secrets to Yoshinobu Gakuganji as "a curse."
Yaga explains that a person's physical information can be used to replicate their soul information and then placed into a Cursed Corpse. A normal Cursed Corpse is usually an inanimate object that is programmed to complete a setlist of given tasks and lacks self-awareness or its own cursed energy. This is the most common version of the Cursed Corpses.
Yaga, however, has been able to develop a type of Cursed Corpse that has self-awareness, and he's been able to replicate the results several times. He did so by placing three compatible souls into the core of a Cursed Corpse along with physical information. By doing so, each soul could observe the others and stabilize to the point of achieving sentience. After three months, it would be able to produce its own cursed energy, no longer feeding off that of the creator. This is how Panda, an Abrupt-Mutated Cursed Corpse, was created. His three cores are what allow him to change his physical shape and abilities.
Yaga also replicated the soul information of Atsuya's nephew in the form of Takeru, a small dog-like puppet that lives with other sentient corpses in a forest. There are other dolls, like Cathy and Tsukamoto, who are used for training and dolls used for surveillance and security, as seen during the Shibuya Incident Arc.
Yaga isn't the only Cursed Corpse user in the series. Kokichi Muta is another who primarily uses mech-style corpses as proxies. He's able to control multiple puppets at great distances, but unlike Yaga, they do not have self-awareness. Rather, they are an extension of his own consciousness as he's able to see and interact with others through them. He's also able to transmit his cursed energy to these proxies and use them to perform a number of techniques.
While Kokichi is able to control a great number of puppets at once, they rely on his cursed energy to be able to move and do things. This is where Yaga's technique is slightly superior, as he's able to create dolls that don't rely on his cursed energy. If anything happens to Kokichi that would render him unable to control his puppets, then that's it. Yaga's dolls, however, have their own cursed energy and consciousness and are thus able to continue their missions even after Yaga has been incapacitated.
Yaga's technique is so powerful, in fact, that he was deemed a danger by his superiors after the Shibuya Incident. It was feared that he could create a corpse army and try to take over Japan. They wanted to place an indefinite restraint on him, but needed to know how he was able to create these sentient puppets.
At first, he claimed that he didn't know how he created Panda and that Panda was the only one, but this was a lie. In the end, he is defeated by Gakuganji in battle, and as he lays dying, Yaga tells Gakuganji the secret of Panda's creation. When asked why he didn't tell before the battle, Yaga ominously says "it's a curse from me to you."