objective reality is created when 2 subjective realities intersect.
when you are asleep, or when you are in the womb; you are in your own world, your own personal reality made up of your beliefs and desires. (these are the most basic substances of reality) imagine that your universe is the first circle of the flower of life.
when you awake you enter the collective reality. everyones personal circle 'tuning in' to the same level so that they intersect.
when you are born your world intersects with your mothers world, your beliefs are modeled by her beliefs. then your father, then other family, then the friends you make, you people you meet; all the relationships you have slowly build your reality as you grow. (this is what tantra is all about,not sex,relationships,and how minds and souls link together and change each other)
imagine 2 people with a 'shared delusion', an outsider looking at them sees crazy people, because they are looking from their own reality, but in their world everything they do makes sense, and their experiences will justify their beliefs.
or imagine 2 children playing an imaginative game together, they are sharing an objective reality together which they both influence, but an outsider would just see 2 kids running around.
i dont know if im illustrating these concepts well enough here but its pretty basic metaphysics.
half the mandela effect stuff i see is so dumb though, really obvious disinformation and i dont know why people take it so seriously, and i dont know why folk who have knowledge on this board arent educating people