How to get rid of startup apps ?

Items that start when the Mac starts up can be in:

/Library/LaunchDaemons

Items that start when any user logs in can be found in

/Library/LaunchAgents

or a users ~/Library/LaunchAgents

or as already been mentioned, the user Login Items.

Apple startup items are in

/System/Library/LaunchAgents

/System/Library/LaunchDaemons

Delete the whole .plist.

Usually it won't affect the app. However, there are some that require their background processes to be running.

Some things will need those processes to function. However, sometimes it is just fluff, like with Garmin.

Their Software Engineers are idiots and create a status menu item which serves zero purpose. If you disable that menu item, the app won't run.

You could just move the plist to your Desktop so you can test. It may be owned by root if it is in a system folder. Get Info on it first and see what the permissions are. Note them down.

Once you move it out of the folder, it may pick up your permissions. If it doesn't work without it, you can move the copy back to where it was and reset the permissions using Get Info.

Check menu

 > System Preferences... > Users & Groups > your login > Login Items ???