The ThisWorkbook module is a class module that implements the WorkbookEvents interface, which means you get to select what looks like a hidden Workbook field in the left code pane dropdown:

As if ThisWorkbook had this code implicitly written for you:
Private WithEvents Workbook As Excel.Workbook
Selecting Workbook from the left dropdown will fill up the right dropdown with all events available in a Workbook object:

When you pick an event in there, the VBE generates a stub event handler procedure for you, or navigates to it if it already exists.
Typing the event handler signature by hand will also work, but for the more complex signatures that take parameters of specific types in a specific order, it's simpler to have the VBE generate the stub for you - automatically generated handlers will always have the correct signature!
Workbook_Opensub