why is jquery source code stored in multiple files ??

why is jquery source code stored in multiple files ??

hey guys , bit of a random question , i was just checking out the jquery project hosted on github here .  , and there are so many folders and sub folder and some methods are in one file and the others are in another , now i taught jquery was just going to be one bit js file , the last time i put the CDN link in the browser , i saw a js file that spanned about the size of 10000+ lines of code , so what is this new style of organizing code ? 

is there any document that makes sense of this style of organizing code ?