You can put them wherever you like.  Some put them on an external drive.  I have mine sitting in a folder called "External Runtimes" inside "My Documents" as well as my spare D Drive.

As for installing / unzipping files, I never do that directly to the runtime folders. I always direct the .exe file or the .zip file to deposit the files into a folder of my choosing on my desktop. That way I can go in and look and move things around if needed.  Not all files are packaged into proper runtime structure and not all people package their files up with congruent names.  What they name a folder inside the poses folder, may not be the same as the folder that is deposited into the props or hand or camera folders.  So I go in and rename all newly deposited folders so that they have the same name and I know that they belong together as a package.  Then when I look for something I can look in all of the folders in the library and if I see a folder with the same name in the character, poses and props, I know that those files all make up a package and I look into them to see if there is anything in there that I may want to add to the scene. It makes life so much easier to find things when common folders are named the same.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi