Some on the map, and some in local knowledge. My first apartment, after moving out, is known in the town as “mulkkulinna” or about translated ‘cock castle’. (Not the rooster kind) you won’t find that on a map.
I recall hiking and fishing the Savukoski wilderness and there a place called “vittukeinon peskihaara” or “female genitalia method’s reindeer skirts’ split” and it was, likely still is, on the map.