Can someone answer my questions about Stranger Things?

Pls dont spoil past ep 5 season 3 I havent finished it yet! I understand each piece separately but how does it all connect? So El is the result of tests done on babies that gave them powers. And the upside down has been the subject of govt tests to open it. But why does El have a connection to the upside down? Why... show more Pls dont spoil past ep 5 season 3 I havent finished it yet! I understand each piece separately but how does it all connect? So El is the result of tests done on babies that gave them powers. And the upside down has been the subject of govt tests to open it. But why does El have a connection to the upside down? Why can she close it? Do the other mutant kids have a connection to the upside down? Was she made to open the upside down?if so, How did her "Father" know that testing on fetuses would help him access upside down?
Bonus question how does the upside down work? It has the same cars and buildings as normal world, but when those cars move the ones in upside down obvs dont move. So what's the rule? If upside down is a reflection, how does it react to changes in normal world? When a normal building is demolished will its upside down counterpart crumble?
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