Mobile Device Detection Technology -
Mobile Device Detection (or 'Device Detection') is a process of identifying the type of mobile device or other device visiting a web site or other service. This commonly uses the HTTP headers transmitted by the user's browser such as User agent, though it can also be performed from other headers if present, such as IMEI or UAProf links.
Mobile device detection systems typically combine a detection with the creation or retrieval of a capability profile of the device. These vary greatly in complexity, ranging from one data point to many hundreds. A device detection process answers the twin questions of "what is it?" and "what can it do?".
The HTTP Header - the HTTP header fields can contain a lot of information about the page (see a list here), and one of those fields is titled "User agent".
HTTP user agent header field - The field of the HTTP header which contains information about the device and the operating system making the request to display the bank's website.
Example of a desktop computer user agent field:
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/21.0
As you can see from the above example, it's a computer running Linux x86, so no problem sending the full version bank website. But what about:
Example of a mobile phone user agent field:
Mozilla/5.0 (Linux; Android 4.0.4; Galaxy Nexus Build/IMM76B) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.133 Mobile Safari/535.19
For the above, the banking website would need to be modified to match the Android operating system, and the screen size of the Galaxy Nexus.
A list of other user agent fields for common devices/operating systems:
https://developer.chrome.com/multidevice/user-agent
Simple Techniques To Redirect
The following is not necessarily how the big bank websites would do it, but it gives you a good idea that it's just all about re-directing to the appropriate pages depending on the screen resolution and operating system.
1. Redirect to various html pages, one for iOS, one for Android etc. using php
2. Redirect to various CSS stylesheets, one for iOS, one for Android etc. using php
See the following video starting at around the 3:00 mark:
https://www.youtube.com/watch?v=RvYU9n02Rck
Device Description Repositories -
(First of all, a "repository" is just a place where you collect a bunch of stuff.)
(More wikipedia...)
The Device Description Repository (DDR) is a concept proposed by the World Wide Web Consortium. The DDR is supported by a standard interface and an initial core vocabulary of device properties. Implementations of the proposed repository are expected to contain information about Web-enabled devices (particularly mobile devices).
Authors of Web content - like those who make the bank's website - will be able to make use of repositories to adapt their content to best suit the requesting device. This will facilitate the interaction and viewing of Web pages across devices with widely varying capabilities.
Information in a repository should include information such as