m.subdomain - am I on the right track of understanding?

I have a question, but not sure if I understand it right, so here it goes.

For example, take google.com; If you browse to google.com on your smartphone it navigates to m.google.com. This I understand can be done by detecting device widths etc., but does that mean that Google re-wrote the page for mobile devices or can you treat the m.subdomain like a ghost type domain?

I mean, you can navigate to m.google.com but it really is just google.com with a different CSS or whatever - the index page is exactly the same.

I guess what I am getting at is - if I make a change in an index or any page in the main domain - will I need to update the subdomain separately if I decide to use a m.subdomain setup?

Hope it makes sense.

There are two ways to solve this:

• m.example.com is a new and separate website that needs updating whenever you update www.example.com
• m.example.com is an alias of www.example.com. Your site at www.example.com needs to serve something different (e.g the CSS or even different template) whenever the http request is for m.example.com