(Almost*) around the beginning of the time of iPhone and Android OS app development, the most common way of making a mobile application was by using the software development kit (SDK) from the creator of the operating system (OS).
People who wanted to develop for the iPhone had to buy a Mac (if they didn't already have one), buy an iPhone, and install Apple's xCode development tools. Their option for programming the app was Objective-C.
People who wanted to develop for the Android OS had only slightly more flexibility. They could use any system capable of packing the apk from the Android SDK. You still needed a computer capable of installing the necessary SDK software, at least one Android device to test on, and you had to program in Android's supported Java programming language.