The files are going to be uploaded to wherever you tell them to be uploaded
If your doing the "bare minimum" my guess is they are going to end up in the root directory of your application.
if you look at this thread: https://coderanch.com/t/50781/Struts/File-upload-struts I have a snippet of code that writes a file to particular directory (defined by variables, so you can make it whatever directory you want, creating it if necessary).
If you want to store the file in a database, I'm not entirely sure how to do that, something I wouldn't mind knowing (especially if it were abstract enough to apply to any database that suppors binary fields?).