Miniaturization technology has advanced enough that they don't need fixed installations. Low altitude anti-air defense is probably a bunch of Stingers stored in a locker in key target buildings. High-altitude ABM defense would be Patriots and THAADs, both of which are mounted on mobile trucks which presumably are housed in U.S. military bases and in secret garages or other locations. It's to your advantage to not let your enemies know where your defenses are; pretty much all missiles since the 1980s have been mounted on trucks that let them be stationed anywhere a road goes to.