Metals work as antennas, they absorb photons and convert them into electrons. Metal buildings are usually grounded to Earth, so the electrons are then fed into the Earth. Those photons were signals to and from your quad and also from the GPS system in space.
If a metal structure is between you and your satellites and between your controller and your quad, needed signals will be absorbed by the metal and lost to ground. The closer you are to a building, the more radio signals will be absorbed. Even trees with leaves will do the same thing, which is why you need line of sight with your quad to fly it. Tall buildings often have a metal frame inside them. Also, certain weather conditions with high humidity can absorb the radio signals.
There will always be those weird circumstances where a building is so designed as to act as a repeater or reflector of the signal wave, rather than an absorber. This can give a false sense of security and as soon as the quad moves out of the narrow window of the refocused radio signal, it will suddenly lose all contact. Similarly, there will be days when the ionosphere directly overhead of the operator is highly charged and reflects signals back to the quad and allows for an unusually long distance flight. Then at a different time of day the effect is lost and you can't fly except for line of sight.
Radio waves are not homogeneous due to the many factors just mentioned, but in nearly all cases, they are clear enough when you have direct line of sight of your quad and there are no nearby structures such as buildings, metal structures, and trees.
