Use Route, Course, or Track to navigate in Mini2 from a downloaded GPX hike?

.gpx files are not "map files". When used for tracks or routes, a .gpx file contains an ordered sequence of points. Technically, there is a difference in the representation of a track vs. a route - but it's mostly a technicality. The important thing is the sequence of points, which are recorded the same way in both cases.

A .gpx file may also contain waypoints. Each waypoint has a single set of coordinates, usually an altitude, and some descriptive information. Most GPS devices can navigate to a waypoint. Not all devices will properly load track, route, and waypoint information from a single .gpx file.

Depending on device capabilities, BOTH tracks and routes can be used for navigation. (See below for the pitiful case for the M2.)

A course is a Garmin perversion. The ONLY difference between a route and a course if the file format. Routes are stored in .gpx files. .gpx is a universal text format understood by virtually all GPS devices. Courses are stored in .fit files. .fit is a binary format understood only by (some) Garmin devices. Why use .fit? Who knows? In fairness, when generated by a wearable device with sensors, the .fit format may contain additional information such as heart rate. But no additional information about navigation.

An activity is a track recorded in a .fit file. On an iR device that supports BOTH "sent" iR track points AND "logged points", the "logged points" are typically (now) recorded as activities. If the iR device allows independent control over "sending" and "logging", the logging will typically be exposed in the UI as "recording" or "activity recording". Some iR devices (not including the M2) can actually record the activity in both .gpx and .fit formats.

This article describes the differences between courses and routes, among other things.

https://support.garmin.com/en-US/?faq=uJZh4J9rF20RVCRNfCtXV9&productID=765374&searchQuery=navigation&tab=topics&textPage=5

This one says that routes and tracks are automatically converted to courses when sync-d to an M2. Which pretty much makes the whole question of "what can I navigate on the M2" academic.

https://support.garmin.com/en-US/?faq=ab6qTtjX3j3GgQULbmHhx6&productID=765374&searchQuery=navigation&tab=topics&textPage=9

This KB article gives a good description of controlling send interval, logging interval, and so forth on the M2:

https://support.garmin.com/en-US/?faq=bHHxYUy5b550KiUUP1LXb9&productID=765374&tab=topics&topicTag=region_mapsharetracking

Note that you do not have explicit control over the logging interval. (You cannot say something like "log every 15 seconds".) You can only select "standard" or "high detail", YMMV. Also note that you must explicitly save the activity when you stop recording. And that it only lives on the device until you sync.

According to this article, the M2 can only navigate using waypoints, courses, or activities. Waypoints and courses can be sync-d to the device. IIRC, activities do not sync to the device (only from it to the servers). But see above about conversion of routes to courses.

https://support.garmin.com/en-US/?faq=gfSGYxKR3H9BOfbNrCaJG8&productID=765374&searchQuery=navigation&tab=topics

AFAIK, the only way to create a course AS a course is to do it in the Explore mobile app. Why? Because Garmin likes to confuse things. Don't get me started. :-D

Bottom line: The M2 navigates courses (only). However, tracks and routes are converted to courses when sync-d to the device. So you can start with anything you want.

Tracks and routes in GPX format can be manually loaded to the MINI 2/Garmin/NewFiles/ directory, and they will automatically be imported and converted to FIT format when the device is next powered on. No need for any external software.