Wikipedia entry on Android
Wikipedia entry on iOS
Wikipedia entry on Xiaomi
Wikipedia entry on Windows Phone
Wikipedia entry on Blackberry | and new Blackberry Priv
Wikipedia entry on Amazon Fire Phone
You may find this NYT article on the Amazon Fire phone interesting.
You may find Microsoft's strategy deck interesting as well, and a current update on how their mobile business is faring.
Consider these questions:
Submit your short answer (100-200 words) to either Question 3 or Question 4. (NB: if you answered my question about Amazon before I edited these study questions, that's fine...no worries.)