I have been particularly told to use iframe for a site with which I need to download some images and display them in the application. So, I just wanted to know can we set a default download location for the content to be downloaded so that I can fetch the data from that particular folder directly.
Mobile operating systems don't have an explicit filesystem (that does not mean there is no filesystem underneath the application, only that those OSes try to hide things like a filesystem from the typical application). On top of that, you are running inside a "webview" which is, essentially, an embedded browser. This means your code is running inside a runtime that does not have the concept of a filesystem, which is running on an OS that wants to hide the filesystem from you.
You can explore the filesystem, but if you want the webview to do all of this automagically for you you're going to have to live with the typical caching of files that browsers and webviews do. If that does not work for you, you might have to perform some explicit AJAX calls to get the files you want and store them in an explicit location. To read and write to the filesystem you can use the Cordova File plugin: https://github.com/apache/cordova-plugin-file
Ultimately, what you decide to do depends on how you want to handle those images, so there is no single "right" answer to this question.