there are too many
Gnome, unity, kde, ....
Not sure what your question is asking, other than the GUI is displayed by the desktop manager.
Access to reviews of Desktop Linux can be found at Desktop Linux Reviews and Extreme Tech online on their websites. You can also access reviews in YouTube videos and in popular online and paper magazines such as Linux Magazine.
Yes, your desktop manager can be rearranged and even radically altered. There are programs like RainMeter that allow you to make a custom desktop and desktop manager.
The courses available for Linux include Linux server, Linux desktop, bash shell, and many others.
i assume by gui, i assume you mean the window manager. gnome and kde are popular. i personally like to use xfce, the "cholesterol free desktop environment".
A package manager is a utility to install remove, upgrade, and track the dependencies of software in Linux.
There is no set file manager in linux. Examples of Linux file managers include but: Nautilus (GNOME) Thunar (XFCE) Dolphin (KDE)
Windows is better for desktop computers. Linux is better for servers.
Package Manager is The Biggest Achievement of Linux
The device manager UDEV manages device nodes in /dev in Linux. It is a generic kernal device manager originally was introduced in Linux 2.5, and is still in the current version of Linux.
For one: The Linux kernel. Otherwise it wouldn't be a Linux distribution. That's about it, actually. However, extremely common components include the GNU toolchain including glibc, desktop distributions will invariably deploy the Xorg server on top of which a desktop environment or window manager will run. As far as init systems go, System V is by far the most common, however systemd is starting to replace it.
It depends on which off-shoot you wish to follow from the linux kernel - I favour Ubuntu 14.04 (Trusty Tahr) on at least two desktop computers, and Linux Mint 16 (Petra) on another desktop.