Where are the resource (/res) files used by an app stored? I'd like to know where in the file system the /res folder of an APK archive is unpacked upon installation?

If the /res folder is not unpacked upon installation, I'd like to know how an application is able to access the resources in this folder? For instance, does the system unzip the APK file upon execution?
A signed apk is a  zip aligned file. You can get the apk file on your computer, rename it to zip and try a zip file manager to extract content. If the programmer chose to use encryption algorithms for resources and embedded decryption inside application code, even you are good on reverse decompile, you can get "era" or bigger time to decrypt this or you may never be successfully. Some programmers chose to put files in some "fake" extension file, for example, rename an executable as .png file, so for trivial user it should be enough to not touch that file :) Just practice and remember reverse decompile is not the worse part of programmer life, it is the poor. As for your question, think tha apk as an executable and think for resources as loaded files in memory. In this way, you may never find explicit resources files written on device. As for memory dump, you should know the information there is very scrambled, so making a dump does not guarantee you get clear resources (files). I think what I wrote is enough to answer your question.