It comes with built in maps - the Topoactive Europe maps in Europe for example. When in the hiking activity, scroll through the datafields screens with the up and down keys till you get to the map screen
If you want more hiking specific with more trails, there's maps like TalkyToasters and OSM (free) you can add to the watch
Depending on where you're from, http://www.openfietsmap.nl/ might be something for you. It's OSM based, but is not focussed on cars, but bikes and hikers/trailers. Some parts of the world have very good coverage in there (Netherlands, Germany, Belgium, Alps region), other parts are somewhat lacking, YMMV.
Last night I figured out how to download hiking trail maps from AllTrails.com and upload them onto my Fenix 6 Sapphire via the Garmin Connect Application. Not the most intuitive operation, but thanks to the documentation of others who have the know-how, I was able to complete it successfully. Now I can access these hiking trail maps through the Hiking Activity on my Garmin.
I use https://www.freizeitkarte-osm.de/ they come in different flavors and different regions. They're good, even though I sometimes disagree on their zoom priorities.
Just checked them too. Compared to the one I reported below, much older maps (almost two months old instead of days), much lower contrast, less info for rivers and mountain icons/names. This at least for my region in Italy.
Userbeam is probably less elegant but much more useful for quick glances on a watch.
