Apple makes removing startup items a more complicated process than it likely needs to be. But if you have some annoying windows popping up on your Mac when you log on, you may need to look in your (often hidden) Library folders. Launch Agents, Launch Daemons, and StartupItems can cause your device to automatically load items every time. Just be careful to not change System files that keep your Mac running properly. Alternatively, check under System Preferences -> Users & Groups -> Login Items tab. You may see apps, files, or folders that open when the associated user logs on.