How do exploration maps work and where can I get them?

The map basically works like any real life GPS. You see yourself and your target. Just move towards your target. It will help knowing where you look at, since the map is displayed with the North in the upper direction.

The costs may vary, but they should cost a large amount of Emeralds (15-30?) and one Compass. In survival, I'm certain they can only be obtained from cartographer villagers.

An ocean explorer map can be bought from a cartographer villager for 12–20 emeralds and a compass, and a woodland explorer map can be bought from a cartographer for 16-28 emeralds and a compass, as their 4th trade. I guess that means that we have to find an unexplored village to find cartographers