Local Copies

Pressing "Yes" on this message box will save a copy of the add-in in the following folder:
These add-ins do not appear in the registry as they are loaded automatically.
This folder is also the default folder path when you save a workbook as an Excel add-in.

Excel 365 - C:\Users\"user name"\App Data\Roaming\Microsoft\Addins\ 
Excel 2021 - C:\Users\"user name"\App Data\Roaming\Microsoft\Addins\ 
Excel 2019 - C:\Users\"user name"\App Data\Roaming\Microsoft\Addins\ 
Excel 2016 - C:\Users\"user name"\App Data\Roaming\Microsoft\Addins\ 

Copying the add-in locally

If you are browsing to an add-in that is not saved on your local C drive then you will see the following message as soon as you press OK asking if you want to save a copy of this add-in locally.
You should always select "No" to prevent saving multiple copies of the same add-in.

Press NO and then your add-in will then be added to the list (in alphabetical order).
Make sure that the check box is ticked and then press "OK".