Apple has found itself over the years to be able to build an ecosystem of many different products. Developing applications for all of these different products allows you to gain a consumer base using Apple's products. Go over each of these Apple platforms to find where you fit in being a developer for.
iOS is the platform in which the iPhone, iPod Touch, and iPad all use. Getting to learn iOS development means that you'll be able to support a platform where millions of people all across the world can gain access to your product. This mobile platform is known for all types of apps from productivity, media streaming, games, and more. Support iOS if you're looking for a huge consumer base of mobile users.
watchOS is the platform used for Apple Watches. By developing for watchOS, you can use develop for the unique features of the watch without just making it a partner app of the iOS version, such as being able to get the user's heart rate, motion, and location that is used for physical fitness apps. Build an app for watchOS if you're interested in smart watches or are just building an app that requires physical activity.
tvOS is the operating system that Apple has used for its Apple TV products since the 4th generation came out in 2015. This operating system essentially is a version of iOS built specifically for the TV experience. Developers can implement features that are system-wide to help make it easier to watch media. Create for tvOS if you have a media streaming app that you feel would fit on the Apple TV platform.
macOS is their longest running operating system, previously known as Mac OS X and OS X. Since 2001, Apple has used it for their entire line of computers whether it be their home or laptop models. Every year or two, Apple releases a major update for this operating system that has its own names. The original batch of updates that ran from 2001 to 2012 has been named after large cats. Afterward, the company decided to name their updates after locations in California with the last release being Mojave in September 2018.
In the past, developers of programs for macOS could distribute their releases similarly to any other Windows program of time whether it be buying a license from a website or picking up a boxed copy at the store. Today, many of the developers support the Mac App Store which is similar to their app stores of other operating systems. Support macOS if you're interested in developing for the second biggest desktop operating system in the world.
As a company, Apple has spent many years trying to cover their operating systems together to be closer. This has worked for the most part, but you still need to understand the differences and features before starting to develop. By building a version of your services to each Apple platform, you can gain a large customer base.