The locator bar provides the current direction to any player we want. We can even identify which player corresponds to which waypoint, since the waypoint UUID is just the first 32 bits of the full UUID (the first 8 characters). This means that with a simple mod, you can reliably target a specific player.

EDIT: A simple plugin which obfuscates the uuid of the waypoints, fixes the issue where we can find out which waypoint belong to whom. https://www.spigotmc.org/resources/waypointobfuscator.128111/

How do you find the exact location of someone?

By triangulation.

All you need is the 2d direction vector provided from waypoint, then take another reading from a location alot more far away (in my testing, you really do have to move a significant amount, you might have good accuracy at close distances but when you start going at like 1k+, you have to start moving alot). Using the X and Z coordinates of both points, you can apply triangulation to solve for the X and Z coordinates of the target player.

This works best on AFK players, as there is no change in location so the directions wont be messed up, and even with the accuracy not being exact like 97% of the times, this will most likely if done correctly reveal the area the player is currently in.

Have fun!