It’s free in the sense that there are projects where as long as you are operating an antenna feeding data, you get access to a feed of all data for free with the only restriction being you can’t compete by just restreaming the complete feed.

Oh that's interesting, thanks. So I guess it's similar to ADS-B exchange[1] and the like but for ships.

Have you operated an antenna, by chance? I've considered building an ADS-B receiver to satisfy my curiosity about the planes I see out my window all day.

[1] https://www.adsbexchange.com/

If you want to try the ADS-B receiver, you can do it for < $40. It's out of stock currently, but last year I bought the first one on this page https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

I got busy with other things so haven't done much with it, but there are some Python libraries that made ADS-B easy.