Where is the file where you can change the desktop background?

https://github.com/shalva97/kde-configuration-files

I found user config, but not default.
~/.config/plasma-org.kde.plasma.desktop-appletsrc

Why would you wanna go look in some file, especially a config rc, to change the background?! I have absolutely no idea what you are trying to do. If you want to change your background, just right click the desktop>configure background. Or get a wallpaper service menu to work in dolphin. And just don't touch that particular rc. That's the sum of what you manually do to your desktop, through the settings.