Where do they get this data from?

It's probably just logging AIS data ( https://en.wikipedia.org/wiki/Automatic_Identification_Syste... ). Large ships are required to broadcast (like, on the radio) their position, heading, and so on. Anyone with a radio receiver can collect this data for ships within radio range. Anyone with a large number of internet-connected radio receivers can collect this data over a large area.

Amateur AIS receiver networks are only covering near-shore areas, for obvious reasons.

Covering deep sea/ocean areas (often the most interesting part) requires some paid subscriptions.

They often give out free receievers to spread their network

AIS receivers on satellites.

And perhaps the international space station ? https://www.esa.int/Our_Activities/Space_Engineering_Technol...

AIS transponders are required for almost all commercial vessels

We have ubar for that https://brawersoftware.com/products/ubar