Normally these agencies “discover” these back doors through admitted confession by those bad actors. Tell us how you did it and we’ll reduce the sentence, kind of thing. Sometimes it’s of their own accord if they are high up enough (NSA) but the majority of law enforcement relies on vendors who use the same exploits as the nefarious bad actors. Only “for good”… the bad actors are always a step ahead and continue to give talks on how badly insecure our pocket computers really are.