How powerful is unity for Android?

I have near to no knowledge about android so instead of starting from scratch I was wondering if I concentrate over developing apps in unity can I get close to developing decent apps

Android Studio is dedicated to developing android apps so taking that as scale how close can an app developed in Unity get to an app developed via android studio

Also is it true changing few resources in code and application developed in Unity is good to go for android as well as in iOS or will it be a complete change of code

You decide Made With Unity | Unity

As for your second question, gameplay scripting does not change at all.