I think that McLuhan is talking about two things when he speaks of the “medium is the message”- simply the SPEED and the VARIETY (or, rather, the ABUNDANCE) of the information transmitted from one place or person to another. Since media are, as he says “extensions of ourselves”,
In the past several centuries as the world is inventing new technologies to communicate, from books and newspapers (the old print world) to telephone, radio, recorded audio, film, television (mass media) to what we have now- the technologies centered around the personal computer and the internet (fax, mobile phones, podcasts, laptops, etc.) these technologies are methods of communicating and for the most part, they don’t go away. They are added to or altered to fit the new times. Today’s text messages are the telegrams of 50 years ago, they are just transmitted on an individual scale rather than through a physical agency. Since all these new technologies are being added to the fabric of the media, today we have a media that can reference each other, reference each other with a wider variety of methods. An informal example- books we read today, or for that matter, conversations we have can contain slang or a writing style reminiscent of the technical jargon of movies or TV. We can describe things happening in a sequence by literally saying “cut to…” or “flash back to…” or “Meanwhile, back at the ranch…” like a TV director in a control room can even though we maybe be only telling the story of a busy trip or a wild weekend that happened to us personally. Conversations or books in the past didn’t use such literary devices since these devices literally did not exist. I believe this is one such way we can state that media is embedded within other media, the interconnection of all these media.