If you are on a Mac and wondering how to transfer files into your Garmin eg a Forerunner 965 - no, the horrible Garmin Express is not the way - you should be introduced to the wonderful OpenMTP.
As the name suggests, this would be the recommended way to transfer files for (nearly) all MTP devices.
Leaving this here as there are scant documentation on how to do this if you are not running Windows or Linux.