In case anyone else runs into this issue, I’d forgotten about the Normal.dotm file, which I should have remembered from my Word for Windows days. It stores all your customizations.
A very helpful reddit user also told me where to find the hidden folder where it and my other user-created template files are stored, so it was easy to transfer them to my new Mac:
~/Library/Group Containers/UBF8T346G9.Office/User Content/Templates