Where are the Excel templates located on OS 10.15?

~ tilde indicates hidden Local Library

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Method 2:

1. Go to the Finder (or desktop).
2. Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
3. With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.

 4 Select Library from the Go menu (while holding down Option) to access the hidden folder.

Likely in a hidden folder...

macos - Where are Microsoft Office for Mac My Templates stored? - Ask Different (stackexchange.com)

Any templates for Office 365/2019/2016 should be here:

/Users/your_account_name/Library/Group Containers/UBF8T346G9.Office/User Content/Templates

Great news, take care my friend! :)