[dutchmartin]

Modern vessels are required to have a AIS transponder. This is a radio that sends the ships speed, position, angle and optionally their destination around by marine (VHF) radio. These maps are probably made by having thousands of receivers recording the AIS data.

[pgorczak]

There are satellite networks that can pick up AIS broadcasts from almost anywhere on the planet. So you can track a ship in the middle of the ocean with a few minutes delay. Pretty amazing to imagine how many signals these satellites have to process at once and how weak they must be after making it all the way into space.