jquery class selector getElementsByClassName

jquery class selector getElementsByClassName

Hi all,

I know that jquery is the library of javascript. 

In jquery, I had the script using the class selector ( $(.class123) ) which is work well in IE8. In the official website, it mention that it would support for IE6+.

But when I use  getElementsByClassName in IE8 which is not work. I checked that IE8 is javascript 1.3 and do NOT support  getElementsByClassName. 

Here is the question ?
1.  Do jquery use the javascript native command, such as  getElementByID or     getElementsByClassName behind ?

2. if not, how can jquery class selector to perform the function without using  getElementsByClassName  

3. How can I study or check what is functioning of jquery command in the javascript ?