Any mechanism can be activated remotely - wifi, bluetooth, radio signal, mobile phone call.
The bottom line is to trigger the explosion mechanism through a given signal. The ultimate goal is to supply a sufficiently strong charge of electricity to C4 so that it detonates. Or create a spark in the gas. Or heat up a cylinder of gasoline.
This, in turn, is done by simply closing the "normally open relay" with the subsequent activation of the desired action)