Depending on the plumbing code adopted in your jurisdiction (IRC or UPC) different rules might apply to you plumbing vent
installation. In UPC (Uniform Plumbing Code) all vents must terminate above the roof / above plumbing fixtures. In IRC (International Residential Code) only one (main vent stack) needs to terminate on exterior / IRC also allows for sidewall terminations.
In addition to the above rules there are so called Air Admittance Valves (AAV) that are permitted in some areas and AAV’s must be installed min 4” above the fixture’s drain (for single fixture), and min. 6” above the flood level of highest fixture in branch venting (multiple fixtures in a system).
Again, it depends on your jurisdiction requirements so I can’t give you any definitive answers but in general; a plumbing vent must be always located above any plumbing fixtures in the system.