Hi all!
I want to understand the whole process of starting a graphical user session in Lubuntu 19.10.
I mean, what are the files involved? and in which order they are executed?
I suppose there is a main file that connects them all, like an init script for X. Maybe I’m using wrong terms for concepts, so please bear with me.
I looked into the Manual, but it is about more general usage, and no technicalities.
X is usually started on many if not most distros these days by having systemd start the display manager (sddm in our case). Hopefully that helps get you started…
Hola lobaluna,
You can see all the services coming into play (network, disk, firewall, sound, input devices, power saving…) until loading the display manager (sddm):
$ systemd-analyze blame
When you log in, depending on each distro/desktop configuration, .rc files will start working according to the login type:
github.com/lxqt/lxqt/wiki/ConfigLaunch
linuxjournal.com/content/profiles-and-rc-files
files files files!! 
EDIT:
I’ve seen another thread of yours about Switching to a second user session without closing the first one. So I guess you were trying to figure out how to do it and my comment didn’t help at all.
Providing true user isolation requires the display manager to closely manage the lock session (Hey xscreensaver!). I recommend using another manager (Lighdm/light-locker). SDDM is only a suggestion.
Think the link should be: Checkout all LXQt components at once by using git submodule. https://lxqt.org/
Gracias for your insight, @anon32327672! 
You are really investigative (kudos!)
I finally figured out that using the screensaver was a perfect way to close one sesion and then start a new session in another console VT without passing thru horrible SDDM (yes, it’s light, however its documentation do not help to make a new theme, it lacks some features such as the one commented by you… etc.)
Going back to this question, I really want to understand LXQt way of handling X stuff. Mainly as you discovered previously, I want to know how to have another session (I use this technique when working on something personal and then switching to work stuff, and viceversa: each one of them with a different user).
Your suggestion is a nice command that I was not aware of. Thanks for that, too.
Since I have so little time with LXQt, I’m trying to understand it as it is with SDDM, then I will probably switch to LightDM.
I see that @anon32327672 has corrected the link in his original post - so you can disregard my reply.
sorry if it was confusing.
On the contrary, thanks for helping, leok!