How exactly did the trolls and kids end up in the same session? I know that they traveled through the 4th wall and the meteor, but was it explained how karkat and Kanaya and such ended up physically meeting rose and dave?

Dave and Rose died on their Sacrificial Slabs when the Green Sun was created, and thus kind of floated out of the blazing mass of the Green Sun and stayed around until Sollux sent the Meteor flying in the Green Sun’s direction.