What exactly do you mean? Were you able to modify pages before adding the platform? It should be like this: under your src lay all your pages, components, assets etc. Over there you should modify whatever you want. It’s seperate from any build folders for iOS and Android.
If you want to create a new build (based on your src folder) you should run
ionic build <platform> or
ionic run <platform> if you’re on CLI v^2.2.2
So, you have only one src folder. That’s the place you should modify whatever you want. If you want to create a new build, run one of the mentioned commands. Now it creates a new binary for you to run on your device.