Is that Geto ( the person in his body) wants to merge with Tengen( the immortal guy) to force a new evolution upon humans that's why the two will stay to protect his body in case he comes to merge. They need to stay back bc tengen is closer to a curse than a human and Geto has cursed spirit manipulation. Tengen has the back door to the box Gojo is stuck in but you need the other part to free him. You could do it with only the back door if you had the two other things but gojo either destroyed them or their missing. That means that they have to find this person called the "angel" who can completely destroy? Cursed techniques.
Idk if everything I said was correct but that's what I got from it and I hope it felps.
Wow this is new