Create Word Plugin to Insert remote content in remote SharePoint Library into local Word document

AnzaGen VBA is all that I know, so I googled for VBA access to sharepoint library, which turned up quite a few promising leads.  While most of them are for opening an Excel file that is on sharepoint, the process would be the same for accessing a Word file.

The correct term for what you are referring to as a Plugin is Add-in, which is a macro enabled template (*.dotm) that is saved in the Word startup folder so that the macros in it are available for use with all documents.

To provide a userfriendly access to the macros, you will probably want to make use of a UserForm and Ribbon Customization.  Fot information on those, see the following pages of Greg Maxey's website:

http://gregmaxey.mvps.org/Create_and_employ_a_UserForm.htm

https://gregmaxey.com/word_tip_pages/customize_ribbon_main.html

You need to realise that the implementation of VBA on a Mac is not equivalent to VBA on Windows, so there may be some issues with developing an Add-in that can be used on both platforms.