Where to find app which opens on startup, but it's not in Users & Groups?
When a system is booted, launchd startup process launches daemons and agents in the following system-level locations (based on the property list files):


/System/Library/LaunchDaemons (system-wide, not writeable when SIP mode is enabled)
/Library/LaunchDaemons (system-wide)


When a user logs in, a per-user launchd is started and it loads the user agents from the property list files found in:


/System/Library/LaunchAgents (system-wide, not writeable when SIP mode is enabled)
/Library/LaunchAgents (system-wide)
~/Library/LaunchAgents (user-level)


Source: Creating Launch Daemons and Agents.



On earlier systems, third party startup items could be also found in /etc/rc.local file and /Library/StartupItems folder, but with Mac OS X 10.4, startup items are deprecated in favour of launchd daemons.

Source: Old Systems and Technology - Startup Item.