Can I download OSMAnd maps onto my Garmin Zumo XT?
I am looking for something that shows off-road ATV/UTV trails.
Although you can create maps for OsmAnd and maps for Garmin devices from OSM data (and the default maps on both are actually based on OSM data) the formats aren’t compatible. You can create maps for OsmAnd using OsmAndMapCreator (although most people just download maps created by OsmAnd itself); for Garmin devices, you’d use mkgmap, although again many people just download pre-built maps.
For Garmin, many different “map styles” are available - you’d want to pick (or create) one that highlights the features on ATV trails that you’re interested in.
Pretty sure OSMAnd uses Open Street Maps as its base map.
If you’re wanting to download a track that you’ve created in OSMAnd, you can export it as a GPX, then import the GPX into Garmin BaseCamp, and then sync/upload to your Zumo.