Unix wants to present all of your files in a single hierarchical tree, no matter what device they're on. This makes it possible to name any file on any disk using a single descriptive string, called its path.
In order to do this, when a new disk volume is mounted, whoever mounts it names an existing folder to be the "mount point" for the volume. Whatever was in that folder becomes inaccessible, replaced by whatever is at the top level of the newly mounted disk image. When the disk is later unmounted, the folder and its contents become accessible once again.
Unix permits any folder to be "sacrificed" in this way. OS X is based on Unix, but does not normally avail itself of this flexibility. It prefers to "sacrifice" a newly created empty folder as the mount point. /Volumes is the place where OS X conventionally creates this folder. When the volume is unmounted, OS X also removes the newly exposed and still empty folder.
But that's only an OS X convention. OS X is still Unix under the hood, and Unix apps can still mount volumes on whatever folder they want.
Actually, OS X sometimes does, too. That's how FileVault I works. Your home folder contains only an encrypted sparse bundle disk image. When you log in, your password is used to decrypt and mount the disk image, using your home folder itself as the mount point. The disk image (and anything else in your home folder) become inaccessible, but that's OK. The disk image is already mounted, and doesn't need to be found again. In fact, it's good that it can't be accessed, because that keeps anyone from fiddling with it while the user is logged in. When the user logs out, the disk image is unmounted, and the "real" home folder becomes accessible again.
Finder synthesizes its top-level list from many sources. Obviously the list includes anything that really is at the top level of your boot volume and is not invisible. You know it also throws into the mix any volumes with mount points inside /Volumes. But it doesn't stop there. For example,
hdiutil info lists all volumes mounted from disk images or from RAM disks, no matter whether their mount point is in /Volumes or not. More generally, OS X also inherits from MacOS a method to find info about every mounted volume however it got mounted.
Most of your unexplained items seem to be Google-related. Who knows why, where, or how Google does anything?