Well,the actual details are not made public,but we can speculate...One thing is for sure that the warhead,the launcher and the control systems are kept at separate locations and not in an assembled and ready to use form...There may well be some "Bespoke" involved,that the warhead may be usable only with the certain delivery system..Then the "More than one man" system..in which more than one person have the security codes to access different essential systems to launch a nuclear weapon...
I once watched a documentary on Russian Nuclear missile submarines and this Russian commander was saying that its not easy to sit on a position where you can destroy a country or two by pushing a few buttons and its a huge responsibility....Likewise the guardians of Pakistani nukes are aware that its no child's play