ITF wages are minimum. Lots of mariners are paid a lot more than the minimum.
Some may get sponsored by their employers, whether that be direct Owners, Ship managers or just Crewing agents.
I most European countries all the necessary courses goes into their basic education per STCW requirement, or in countries that has them, Academies, which fully or mostly free.
Refresher courses are usually sponsored for those on permanent employment, but may be required to be current for new hires, seasonal hired crews, or contractors etc.
I assume that the companies. like Seagull, who offer these courses are mainly paid by companies, or through Government sponsored re-education schemes.
Even as a freelance Consultant/Surveyor I have never paid for any courses, such as Survival, HUET, etc. and I have gone through many over the years.
Obviously the ways things are done in the US are very different from Europe and most other places.