The 'cannot stat' message produced by
cp is telling you that those files don't exist. You probably don't have the package installed that contains them. Since I'm not an Arch user, I don't know which package that would be, but I'm sure that Arch has some alternative to Debian's 'dpkg -S /etc/xdg/openbox/menu.xml'.
As for editing text files from terminal, you're probably looking for
nano. If you're more familiar with
vim or
emacs, either of those would work too, but I'm assuming you're not based on the context. One does not simply forget about
vim!
Disclaimer: Some of what I am about to say is Debian specific since I have a heavy background in Debian, not Arch. My hope is that it will guide you in the right direction.
The reason that
startx doesn't do anything by default, even after you have installed LXDE, is because you haven't told it what to do. It doesn't inherently know which desktop environment you want to use or even which ones are available unless you tell it explicitly. While many other distros will at least keep track of which desktop environments are installed through the package manager and attempt to set sane defaults, that is not an elegant approach based on the Arch philosophy, so Arch doesn't provide such a mechanism (that I am aware of).
Your next step should probably be to search the Arch wiki for information related to setting the default desktop environment. In Debian this can be done quite simply using the command 'update-alternatives --config x-session-manager'. Similarly, if you installed a display manager in Debian, such as
lightdm, it would be automatically configured and give you a list of installed desktop environments on next boot. If you wish to change your display manager in Debian, you can run 'dpkg-reconfigure <display_manager_name>'.
Edit: After a quick search based on the above information, I believe that the following two Arch wiki articles should help you. The first one is for setting the default session manager for your user and the second one details setting the default display manager for the system.
https://wiki.archlin...dex.php/Xinitrc
https://wiki.archlin...Display_Manager