macOS: why only few login elements are in System Preferences
The login items that you referenced are for your individual account.  The other “elements” you referred to are for all users and system wide daemons/services respectively.


~/Library/LaunchAgents per individual user
/Library/LaunchAgents for all users
/Library/LaunchDaemons for system wide services regardless of user (run as root)


Extensions are handled the same way, some are user only, while others are assigned to all users.  You can find a list of what’s being loaded with the launchctl command.