OK OK I HAVE A THEORY
What if both Ran and Boo is a time traveller right, and we know that the past Karl’s are there because they got stuck and were there for to long.
What if Ran worked out what was going on but couldn’t convince Boo so he created a portal that joined the dimensions that also created Ranboo, because Boo got stuck in the Inbetween and became the “Keeper” and that what it refers to with “we” it’s referring to Boo and all the past Karl’s.
And of course Ran is in charge of The Other Side, and that’s why Ran said “I’ll see you again” referring to in The Other Side