Are gable vents necessary with can vents?

Gable vents. Gable vents go on the ends of the house where the peak is. At the top point of the triangle.

The materials must be carefully selected for their ability to withstand temperature variations, physical force and be air resistant at the same time. One cannot just take a piece of steel and place...

A volcanic vent is any opening from which gas or molten rock may erupt.

A vent is a pipe that continues upward from a drain and eventually goes out the roof. It is needed on all of the drains to prevent syphons. Check local building codes.

No. You vent air from a sewer line.