How can Apps be still opened even after shutdown?

One possibility is that it didn't actually shut down at all.

If you have an unsaved file in excel or some other program then it can refuse to close the program and cancel shutting down the system.

If you simply click shutdown and then walk away from your computer you may miss the "applications are preventing this computer from shutting down" screen where you have the option to "cancel" the shutdown or "shutdown anyway". If you cancel then you can manually save your data while closing programs.

If you leave the screen it on too long it will simply cancel the shutdown and lock your computer, leaving you at the login screen you would have been at were it a full shutdown.

Before walking away you should make sure your computer is actually properly shutting down. I've seen many people leaving their computers unlocked or insecure because they tried to shutdown with unsaved data in applications.

EDIT: This answer assumed you were on Windows 10 and was posted before the edit. Windows 7 does not have this, but for those that use Windows 10 may use this answer.

Your apps are not already open after shutdown, but windows has a setting, that upon reboot by windows update, it will relaunch programs that were open, and those programs such as word and excel remember what files were open and open those for you too.

It was an effort of Microsoft to make Windows behave more like a mac and make people less frustrated about windows update interrupting workflow.