SEB doesn't perform any real changes in macOS (this is comprehensible as there is no administrator password necessary when starting SEB for macOS the first time).
One global setting (user defaults) change is performed while SEB is running:
The path where screen shots are saved is temporarily changed when SEB starts and set back to the user's preset when SEB is quit.
If the Mac would be force restarted while SEB is running or SEB would crash, starting and quitting SEB resets the screen shot path again to the original user's preset.
In the unlikely case this should ever fail, it is easy to reset this screen shot path to the system's default (the desktop folder) with the following command line in the terminal:
defaults write com.apple.screencapture location ~/Desktop && killall SystemUIServer
If you want to remove SEB settings and SEB completely from your machine, do the following;
Trash Safe Exam Browser Application from your applications folder.
~Library/Preferences and find Safe Exam Browser related
plist file and trash it.