Futaba remotely hacked the cell phone to launch the metaverse app while Sae was holding it and Akechi was nearby. This caused them both to briefly enter the metaverse near Sae's palace. However, the cognitive version of the Police Station interrigation room was far enough away from the "casino" that there were no distortions, so both locations looked identical. Since both Sae and Akechi would expect Joker to be sitting in the Interrogation Room, this resulted in a cognitive duplicate of him to be sitting there.
Thanks and one more thing
how did Akechi understand Mona? I thought they said for you to understand him, you have to hear him speak in the metaverse first. Did Akechi meet mona prior?