The Mac version of Word does not use Building Blocks templates, that's a Windows-only thing.
Word for Mac uses AutoText, where the items are stored in templates attached to the document. For autotext galleries, Word refers to .dotx files that are stored in the application package.
Using the Organizer (Tools>Templates and Add-ins>Organizer), it's possible to copy autotext from one template to another. I don't recommend that you try to add custom content to the autotext galleries included with Word, as those are likely to be over-written in a program update.
Here's my article about how to create custom autotext gallery content in Word for Mac: https://www.brandwares.com/bestpractices/2018/07/xml-hacking-add-a-table-of-contents-to-the-toc-gallery/ The article uses a table of contents as an example, but you can use the same method for cover pages, page numbers, etc.