How come com.google is a valid domain name?
.google actually is a valid top level domain (or top level zone), as is '.youtube'. Google applied for those TLDs a long time ago... successfully as we can now see. Google can now further delegate authority within that zone and com.google and other subdomains (or delegated zones) can become valid and be operated.
Here's a news article on zdnet about this from 2012 and you can find out some .google background info about the WHY at The Register.
The mirror effect 'on page' on the other hand is probably done with a little css3 (as e.g. described here) – I now only quickly checked with firebug and saw that on rollover the elements are actually highlighted on the opposite side.
The How...
In the past few years icann opened up applications for custom GTLD names, you can take a look at application statuses on the ICANN website. Expect to see many new company names as gTLD in the next few years such as .bbc,.foodnetwork,.hilton etc. Sadly these scheme was only for the super rich or big corps with buckets filled with 100 dollar bills.
The Why...?
The cname com. on .google was created on March 31, 2015 and is apart of an 1st Aprils fool joke which in my opinion is a poor joke! a real joke would of been doing it to the main site :)
The When...
The whois records for .google displays:
created 2014-09-04 Thursday, September 4, 2014
changed 2015-01-07 Wednesday, January 7, 2015
It's worth mentioning that Google has registered a vast range of gTLDs including reusable gTLDs through supported registrars. Such domains are .how that is owned by Google and can be registered through big registrar companies such as 123-reg and name.com.
Google's upcoming public domains are:
.dad .esq .here .meme .prof .zip .ads .day .fly .mov .rsvp .boo .eat .foo
You can find out more and keep up to date on Google's Registry Online Frontier website.
Whois'ing...
Just to add most online whois services do not understand whois'ing top level domains especially when it comes to gTLD domains I recommend gWhois.
.google is the TLD.
It's similar to a .com or .net extension so there's no trickery involved.