How to copy files in /Applications/* using Terminal?
Applications that come preinstalled with macOS are protected by SIP.  The Calendar App, being preinstalled falls under this category.  Firefox, for example, wouldn't.

So, anything in 


/Applications/Calendar/..... is protected
/Applications/3rdPartyApp/.... is not



  System Integrity Protection restricts the root user account and limits the actions that the root user can perform on protected parts of the Mac operating system.