The Earth's magnetic field varies in both intensity and inclination angle from place to place, so a pair of those values can act like a coarse coordinate. Experiments using coil systems have exposed naive loggerhead hatchlings to field signatures matching distant points in the North Atlantic, and the hatchlings orient in the direction that would keep them inside the gyre rather than drifting out of it.
That capacity matters because the journey is long. Hatchlings leaving beaches in Florida can spend years circulating in the North Atlantic before appearing off the eastern United States as larger juveniles. Adult females later return to nest in the region where they hatched.
A magnetic map is not the whole story. Wave direction, light and chemical cues all contribute, and the resolution of the magnetic sense appears coarse, enough to find a stretch of coast rather than a particular beach.