I think they explained it pretty well the latest chapter. Basically theyve unconsciously (prolly intentional bcuz of evil aono) been performing a ritual that gave aono kun a physical form. Its only possible because the mc is sharing her heart with him.
To add on, like the Great grandmother said, everything that happened from chapter 1 onwards may have been a destined fate. What are the odds that Aono and MC were able to fulfill all the requirements for the ritual at the same time one of the strongest members of the Watase lineage was alive. It’s interesting how the Great GMA is mentioning fate after MC and the supernatural expert girl came to the same conclusion.
can someone explain to me what the hell is going on I'm like entirely clueless