The most common operating systems used today are Windows, Apple OS X, iOS, Linux, and Android. These systems are integrated in our everyday tech devices such as computers and phones, they provide a platform for applications to run on and many other features.
Growing up I've always been familiar with Windows and Apple. I remember as an elementary student there were Apple computers in our class and at home my father had a Windows computer, so I got a taste of both worlds. It wasn't until my teenage years till I was introduced to iOS around 2011 which caused a shift in how we operate our phones because of how innovative it was at the time.
In conclusion, companies for the most part pull ideas from each other all the time so none of these operating systems are complicated to use if you only used one of them your entire life, they're very similar in some ways.