How are windows phone extensionSDK installed while deploying the application?

I am just curious about this thing, searched for it but couldn't find satisfactory answer hence I decided to ask the question here. If I want to use any ExtensionSDK for my application, I have to install it through my IDE. For example if I want to use SQLite for windows phone i have to install SQLite extension. So my question is what exactly does the extension SDK provide? Actually speaking, in this case, it will provide me with all the functionalities a database provides. Like query operations,storing of files,interacting with the file System etc.. But when I deploy my app on the windows phone then is the extension also installed on the phone at the same time? Or is the extension just set of API to access the functionality of the database and if this is the case then is database pre-installed on the windows phone?