In LibreOffice Calc, the shortcut key used to exit the worksheet (i.e., close the application) is:
Ctrl + Q This will close the entire application. If you just want to close the current document (worksheet) but keep LibreOffice Calc running, you can use:
Ctrl + W
Ctrl + W: Closes the currently active worksheet or document.
Ctrl + Q: Exits the LibreOffice Calc application completely