In Leap 15, some settings are gotten via the main menu and Settings/Configure Desktop; others are gotten via Yast. (Of course, Yast is itself accessible from Settings.) Yet others are found in syscconfig. Is there a design principle that indicates what is to be found where?
Basicaly, the settings of a desktop are just for configuring the desktop of a user. Another user on the same system may have very different settings (well, (s)he may even use a different desktop at all).
Indeed e.g. the KDE settings have also tasks that fall outside the desktop configuration and thus require Superuser access to do them. That is because the desktop designers felt a need for direct access from the desktop to some system configuration eareas. Maybe they are right on other distributions, but openSUSE has YaST and I would advice to use YaST for system tasks.
YaST is typical a tool to configure the system. You experience the main difference in that configuring the system requires Superuser (root) access.
As said, YaST is a system management tool to make it easy to configure the system. All things you can do with YaST can also be done without YaST. By editing the configuration files (mainly in /etc). Many will stress that even when you use a tool like YaST, backgound knowledge of what the tool is doing with which files, etc. can come very handy when trouble arises.
And yes, files in /etc/sysconfig are part of those. The files in /etc/sysconfig have a special format that makes it easy to integrate with YaST (or other system management tools). But you can change them “manualy” (with a text editor) when you want, like all the miriads of other configuration files.