How & when to implement Controlled Impedance.

Hello, I wanted to ask a question on how and when to implement controlled impedance on PCB's. This question stems from the fact that on my PCB, there are many signals that I would want to be as accurate as possible and one technique of improving signal integrity was said controlled impedance. Also, for signals such as RF signals, there are required controlled impedances that must be followed in order for the module to effectively communicate. Also, an important question is how is controlled impedance even implemented, as trace length varies between signals alongside other important aspects of a PCB such as the material used to craft it.