GNU/Linux has one single file hierarchy, whatever the number of
partitions/disks. The so-called "root" partition has the root of that
hierarchy: /. A root partition is necessary. It can be the only partition.
However, we often have more. For instance, a different partition for user
files. The associated sub-hierarchy of files is "mounted", attached at a
point (a directory) of the single file hierarchy. For instance, user files
are in /home. As a consequence, if user files are on a separate partition
(rather than directly in the /home directory of the root partition), its
sub-hierarchy of files is mounted at /home (an empty directory in the root
partition). Installing Trisquel, you can choose a custom partitioning or you
can use the default one (with a separate partition for /home).
The organization of the file hierarchy is kind of arcane:
https://refspecs.linuxfoundation.org/FHS_3.0/fhs/index.html (do not read
that!).
For the moment (and maybe forever), you need not understand it. You should
install "packages". The package manager knows where to install them (well,
it is actually the package itself that knows where its files should be
copied). Read that:
https://trisquel.info/en/wiki/how-software-installation-trisquel-different-windows
An executable file can be launched from a terminal (writing the path to it,
or only its name if the executable is in a directory that aims to contain
executable files) or from a file manager (double-clicking on it). However,
for graphical application, we have "launchers". They are items in a menu.
Essentially every package providing a graphical application includes a
launcher: after installing that package (with the package manager), the
related menu item will appear. You can manage the menu (hide/delete items
you do not use, add your own custom items, etc.) from applications such as
"Main Menu", by default in Trisquel.
A "shell" is a command line interpreter. It is a programming language.
Mastering it is hard. But you do not need to master it to start using
GNU/Linux. Nowadays, you never need to open a terminal to use GNU/Linux! Do
not get me wrong: learning shell's basics (starting with its interactive use,
rather than shell scripting) opens up many possibilities. It is often the
most efficient way to complete a repetitive task. But most users can do
everything they need never opening a terminal.
My answers are kind of vague. I can make them more precise if you want to.
But my point is: nowadays, you need not worry about partionning, system files
or the shell to install and administrate a GNU/Linux system! Try it. You
need not even install GNU/Linux to play with it (including to try installing
applications with the package manager: they are then installed in RAM): we
have live systems! See https://trisquel.info/en/wiki/how-create-liveusb and
https://trisquel.info/en/wiki/starting-installable-live-system
