Plumbing drain and vent question
The larger pipe looks to be your main vent. The other smaller pipes connecting to it are vents serving other fixtures. Codes limit the location on where vents can be connected, typically 6" above the flood level rim of the highest fixture being served and a minimum of 42 above the floor. That is why they are connecting as shown in the photo. The plastic vent which would have been part of a renovation/modification after the other work was completed looks to be connected a bit too low.

Just because it may look odd or look inefficient does not mean that it was done incorrectly. With the exception of the plastic pipe, this looks to have been done to meet the code requirements.