We have probably seen Yuta’s curse technique reversal
Yuta’s cursed technique allows him to copy any other curse technique as long as Rika is able to eat a part of the user’s body. Essentially this means that his technique is acquiring different techniques and adding it to his own. Cursed technique reversal allows a user to do the opposite of what their technique typically does, like how the reversal of Limitless being red and expels everything around it, instead of blue attracting. That is why I believe that Yuta used cursed technique reversal to turn Rika into part of his technique, which would be the opposite of copy giving him a new technique from another person. So instead of “copying” a new technique he instead “pastes” it. Here is my reasoning
Typically a curse technique draws on negative emotion and cannot heal or actually create anything, like Hanami being unable to create actual plants or Jogo’s not being able to create actual flames (when he burned down the restaurant none of the people noticed until the actual bodies combusted from the heat). Also from this example cursed spirits are unable to use reverse cursed technique, due to it destroying their bodies and not needing to since the can heal naturally by manipulating their own curse energy.
Rika is the only example of a curse spirit using reverse cursed technique, with Sukuna actually being more akin to a reincarnated sorcerer. The only other examples of a shikigami using RCT is from Megumi’s own technique, tranquil deer using RCT to heal and Maho using the sword of extermination. However, Rika is described as a curse spirit, even by Gojo who can likely tell with his six eyes. Yet she is able to use RCT without any drawbacks, likely due to her actually being a part of Yuta’s innate technique.
Rika is also necessary for Yuta’s technique, with her eating a body part of another sorcerer being the only way we know that Yuta can gain a new technique in the current day. Rika is the only example of a shikigami being so closely tied to their users technique without it actually being a part of their technique like Ten Shadows. Even with Yuki and Garuda, the only other example, Garuda was described as being more like a curse tool than a shikigami due Yuki being able to use star rage on it.
Yuta has also been shown to be an expert on using reverse curse technique, being able to learn it much faster than even Gojo, who likely had years of experience and almost died to learn it. He is also one of the only sorcerer shown that can use it on others and the only one shown actually outputting it. Due to this I believe that when Rika was hit by the car Yuta subconsciously tried to heal her with RCT, but instead used curse technique reversal and made her a part of his technique. Then after the original Rika passed on he did it again to make the new, fake Rika so that he could have a storage for his curse energy and copied cursed techniques.