Developing a mobile app for a business is an investment – it needs funds and time. Many often companies get apps made for IOS, Android and Windows without doing any market research on what type of phones their target customers actually use. Thus they end up in paying three time the cost as they might have just needed one app. There are cases where the target audience uses mostly one type of phones (like android) and hiring just android application developers can save cost and time. There are some factors that indicate the dominant phone usage by a target audience:
Demography: Developing countries and rural areas usually have android phones more popular . Developed countries and urban areas on the other hand have apple phones more popular. So if the business is focusing on launching an app that are in these areas then it can find studies of what phone types the public is using.
Socio economic factors: Iphones do not have many cheaper variations (they just have outdated older models). Where as in Android phones you can get them at any price range and there are hundreds of companies who are manufacturing them. So they fit in the budget of almost every person (from people with lowest income group to highest). So if the focus of the company is to use the app to offer something to an economic segment that looks for cheaper variations of a product (for whatever reasons) then investing in android application developers could be a good idea.
Market Share: There are areas that are captured by some phone companies where they advertise a lot, have big discounts and people trust them. Those companies could be either apple or android or windows based. Understanding what is your target market influenced by can help in deciding on just one type of app.
There are markets where there are almost equal or good amount of customers for all types of phones could be a good idea.