Do Airlines and Airports recruit Avionics engineers?Or is it only the aircraft manufacturers who recruit them?

Answer 1

As far as I know it is only the aircraft manufacturers. There are also suppliers to the manufacturers and trade associations involved in avionics testing that need those skills. Airlines expect the aircraft they purchase to have functioning avionics and do not plan to design new stuff. Airports have technicians for their ground-based equipment but don't need original designs.

Think of an airport as equivalent to a supermarket. That place is full of food but does not process canned goods inside their store and so doesn't need specialists in how to make that food.