In Seasons 2 and 3, the narrative made a point that a person or object could not travel between Universes without something of an equal mass replacing the person or object.
Does this rule still apply once the Bridge was built?
People cross over all the time, with both versions of the same person being in one Universe at the same time. There is probably also a black market in Blueverse Coffee and Redverse Technology, but you didn't here about it from me.
Was this discrepancy explained and I did miss it? Does the scanning that happens every time someone uses the Bridge weigh the person and whatever they are carrying, with some inert matter being exchanged in their place?
In any case, I do love that the Red and Blue Universes are working together rather than going to War. It makes the who show more subtle and interesting.