Is It Illegal To Have Source Code?

New to this whole world and all the developer licences and rules and what functions Apple will allow or won't, If I want to design an app based around the code of a function already in place, such as airplane mode, is that all closely guarded? Does a developer have to just keep poking at there own code to mimic functions, or I am able to scan Apple's developer pages and say, my app would really soar and be successful if I could only use the Do Not Disturb function.