When people say "In Unix, everything is a file" what they mean is that also things which are not files are treated as files.
Of course most operating systems work with files. Text files, image files, sound files. But not all operating systems treat devices as files. That's an important difference. If I list the content of my /dev/ folder in my Ubuntu operating system (which is Unix based), I get a list of more than 200 devices. Some of those devices are hardware, but are shown inside a folder. For example hard drives, USB ports, mouse and keyboard, audio devices and printers among others. Some of the devices are virtual, for example /dev/urandom, which behaves as an infinite file full of random numbers. It's not a real file on my hard drive.
All of those devices are treated as files. I can read data from and/or write data into those devices. Here there are examples of copying data from different devices into the audio device. This is possible because they are treated as files. The (geeky) result is the ability to listen to the content of the hard drive, the mouse motion, the computer memory or the pixels of an image. This would be much harder to achieve if devices were not treated as files, because each device would require different methods for reading and writing data.
That being said, what "everything" means varies from system to system. For example, OS X is based on Unix, but does not have a /dev/audio device. It uses a proprietary audio system called CoreAudio. So in this case one might say "almost everything is a file". Then, in systems like Windows, where "everything is not a file", you could still do things like copy the content of a file to printer port (typing something like
copy mydocument.txt >lpt1:), which is similar to copying a document to the printer device in Unix systems.
Do other operating systems such as Windows and OS X not operate on files? Yes they do. Windows and OS X operate on files, but Windows does not treat devices as files, which is part of what "everything is a file" means.