If the wall was thick enough to cross behind the 3" stack for the toilet, then it could have revented at 42".
However, most walls are either 2x4 or 2x6, which is not enough space.
Therefore the revent, which can only tie in at 42" above a floor, (at least 6" above the flood level of a fixture) must go to the next floor.
This diagram is standard plumbing in apartments and homes.
For every pipe going down, you also have one going up.
Look at the neighbors home and count how many vents are on the roof.
I normally run between three and eight vents through a roof.