Skype comes as a default app on Windows 10. The default setting of Skype adds itself to the system tray and will be open automatically every time you boot your Windows 10 computer. To stop this, you can go to the Settings menu of Skype.
As you know, having too many apps on the startup can slow down the booting process of your Windows 10 (and other Windows versions). You won’t be able to see Skype on the list when trying to manage startup on Windows 10, be it via Task Manager or via Settings menu. However, if you add this app to system tray, it will continue running as the background process, even if you have closed it or reboot your system.
To stop Skype from starting automatically on Windows 10, you can go to the Settings menu by clicking the three-dot icon next to your profile name and select Settings.
On Settings page, click General on the left panel and slide the button on the Show Skype in the Windows notification area (system tray) option.
From now, Skype won’t run in the background again and it won’t be open automatically every time you boot your Windows 10 computer. Chances are, you will need to do the steps above after receiving a Windows 10 update.