This tropes is one of many examples of how TV and film often depict a lack of realism. This trope also involves another of my interests: technology and in particular Apple.
Everyone Owns a Mac is trope that involves depicting all or most of the characters in a work using Mac computers (and often other Apple products as well). In real life, the vast majority of people use Windows PCs. I myself have used them since childhood, but I have had experience with using Macs at the Apple Store and in school, since I am studying a field that often uses the Mac.
In fact, that may play a role in this trope. Many, if not most, people in the film and TV industries use Macs. Therefore, many of those who involved with creating film and television depict the characters using Macs even if it would be unrealistic for them to do so.
Often, characters that you see using Macs are creative types of people such as filmmakers, musicians, artists, and writers, but of course, you will see all types of characters often using them.
The trope can apply to other Apple products, but not all of them. iPod are still the most popular MP3 players, in the real world, even though the popularity of the iPhone, iPad, and other smartphones and tablets has led to their gradual decline in popularity. Therefore, it would not be unrealistic to depict lots of characters in one work owning and using iPods.
Using iPhones could apply to this trope because in America, at least, the iPhone was available only on the carrier AT&T. In February 2011, it was first made available on Verizon Wireless, then on Sprint in October 2011, and finally, in April 2013 it was made available on T-Mobile and all of the four largest wireless networks carried it. So, while the iPhone is often seen used by TV and film characters, it still may not be realistic for everyone to own an iPhone, though it is becoming more and more realistic, but that still does not even account for other popular smartphones such as the Samsung Galaxy line, for instance, which is one of Apple fiercest competitors of the iPhone and iPad. It may be more realistic for works set in the UK or Canada.
With regard to the iPad, it would not seem so unrealistic to me, as the iPad is the most popular tablet even though there are many competitors that make up a noticeable minority such as the Samsung Galaxy tablets, the Kindle Fire, and the Google Nexus.
Now, here are some examples of this trope.
In Degrassi, the more recent seasons depict all of the characters using iPhones regardless of their financial situations.
Author Stephen King often mentions Macs in books, if his characters use computers.
On Law and Order: SVU, the detectives are regularly seen using Macs, iPhones, and iPads in the courses of their investigations.