FIT files
A FIT file is a GIS data file used by Garmin GPS sport devices and Garmin software. It contains location and time information logged on the GPS device during a run, bike race, or other athletic event. FIT files can be uploaded to the Garmin Connect Web service for personal fitness tracking.
FIT files allow users to track their physical activities over time. For example, users can upload data to Garmin Connect each time they run and review travelled paths on a map and the total distance, duration, and burned calories for each activity.
GPX files
A GPX file is a GPS data file saved in the GPS Exchange format, which is an open standard that can be freely used by GPS programs. It contains longitude and latitude location data that may include waypoints, routes, and tracks. GPX files are saved in XML format, which allows GPS data to be more easily imported and read by multiple programs and web services.
GPX files are commonly used for exchanging GPS location data with others, creating maps, and geocaching. If you come across a GPX file there are a variety of applications that can open it, including Google Earth and GPX Viewer.
GPX files store three types of data:
Waypoint - Includes GPS coordinates of a point. It may also include other descriptive information.
Route - Includes a list of track points, which are waypoints for turn or stage points, that lead to a destination.
Track - Includes a list of points that describe a path
Basecamp
The easiest way to distinguish between the .fit and .gpx file that are both shown when you connect your GPS to the computer are as follows:
Date going forward eg 08-01-2021 - fit file
Date backwards eg 2021-01-08 - gpx file
You can't delete .fit files from the GPS Internal Storage via Basecamp. You can delete them though using the method in the link HERE