Does every Ubuntu question answer apply to it's derivatives? (Xubuntu, Lubuntu, Kubuntu)

I am wondering what the difference between Ubuntu and it's derivatives is besides the desktop interface (GNOME, XFCE, KDE, etc..)

I have used Xubuntu in the past, and seems I have been able to apply solutions for Ubuntu problems on Xubuntu. Which makes me think that Ubuntu and it's derivatives are just the same car, but with a different paint job?

Am I thinking about this correctly?

Calling the different flavors of Ubuntu derivatives explains it all in a nutshell. They are all the same under the hood except for whatever packages are bundled with each desktop environment. Packages in the default Ubuntu repositories can usually be installed on any Ubuntu derivative.

The underneath of all is Ubuntu as @karel has already stated, so if using GNU/shell or anything terminal based or are the core of Ubuntu's inner-workings, they are all identical.

Yes the differences are in the GUI or desktop. If you are changing GUI settings, a command intended for a GTK+ environment may work in GNOME, MATE, Budgie and recent XFCE (thus Ubuntu-Studio), but may not have the same effect on KDE or LXQt (both being Qt based). The reverse will also be true.

Under the desktop or GUI the user is the toolkit in the software stack. GNOME uses the Gimp+GNOME.Toolkit, which is also used by many others. KDE and LXQt however don't use this, instead using Qt, an older toolkit that is owned by a company that lead to the FSF creating GTK+ from the then GTK (Gimp.Toolkit) and creation GNOME.

This toolkit makes a huge difference, especially for programmers or people who to go through the software stack to deal with the OS underneath. GTK+ is easiest in C, where as Qt is written for C++, but the issue is more than just language, and it's something most users can ignore (but may notice with theming and window decorations).

If I look at a solution, I work out where in the software stack it's impacting, and this will tell me whether it'll work in my different Ubuntu flavor. For most 'fixes' they will work, but because not all the stack is identical - not all fixes will help.