I've always used GPS Babel (https://www.gpsbabel.org/) to convert tracks to .gpx but I've found that I could always upload a .gpx track to any gps without any issues using whatever software comes with it.

Northwest Topo will be a map set, not a .gpx track so not sure how the install works for the Fenix.

As Marc said Northwest Topo is just a maps set. Not even a program, and definitely not a gpx file. You need to use some shell programs like Garmin BaseCamp (which is free and supposed to come with your watch on CD / DVD) to view specific maps and gps tracks on that maps. You also will use this program to create points for your watch to use it for navigation. And lastly may be it will make sense for you to learn how to use GPS to get advantage of this tool....