I guess once you have a phone it would not be too difficult to do reverse engineering, but I am still confused with level of vulnerability SMS / WhatUp / IMessa

I guess once you have a phone it would not be too difficult to do reverse engineering, but I am still confused with level of vulnerability SMS / WhatUp / IMessage what does it mean a link other type "unknown" zero-day !? It is very odd, on one side governments complaining about other govs are hacking and safty, and at the same time requesting back-doors from tech giants, all the while tech people are saying "crying at loud you cannot have both at the same time", and this is going on for past 20 years ...