Because the F-35 is highly integrated, it isn’t so easy to describe where the EW system ends and other parts on the electronic architecture start. Everything gets channeled through a central processor that sorts out diverse inputs at the rate of a trillion operations per second, and then the most appropriate on-board systems are used to address threats as needed. For instance, a “distributed aperture system” of six infrared cameras scattered around the airframe might detect surface-to-air missile launches originating from a particular location, leading to a pilot’s decision not only to dispense flares but also jam radars in the same area using the fighter’s multi-function radar.
Executing that kind of complicated response from a legacy fighter would take precious time, and might not be feasible at all given design limitations. Moreover, a legacy fighter would lack the advantage of an integrated stealth design, making it much more vulnerable even with EW upgrades. No aircraft can be invisible in every electromagnetic frequency, but the F-35 is designed to be so hard to detect in the frequencies used by targeting radars that an enemy would need to be nearly within visible range to even attempt a kill (very few enemies would be able to get that close without being shot down).
There are many arcane features of the F-35 EW system that I don’t have space to describe here, such as the towed decoy that distracts incoming missiles and the digital library that stores details about all known threats. Suffice it to say that when you take into account all the electronic features of the F-35 fighter and then combine them with the stealth qualities of engine and airframe, you end up with an invincible combat aircraft piloted by an operator with unprecedented situational awareness. This is why F-35s typically kill over 20 adversary aircraft for every friendly loss in exercises aimed at honing pilot skills.