the map is open-source, but be aware that #Locus as app is not.
IMO this is not necessarily a bad thing, software developers have to earn money.
Another great (not #FOSS) app for navigation during sport activities (#running, #cycling, #trekking, etc.) is IMO #komoot : komoot.com
It also uses #openstreetmap data

Edit: OsmAnd is opensource, thx for telling me