It's possible that they purchased the movies (although definitely without the proper licensing; buying a DVD allows for personal use, not training a commercial model), or maybe they simply pirated them.

It's also possible that models' entire understanding of the aesthetic comes from screenshots of the movie. Even if OpenAI didn't feed in each frame of the movie, they definitely fed in lots of images from the web, some of which were almost certainly movie screenshots.

I guess it’s like

1) Download Ghibli film material from an arrrr site. Extract frames using ffmpeg. Pay someone $1 per week in Nigeria to add metadata for each (or some) frame(s).

2) ???

3) Profit

there are probably millions of stills from their movies on the web.

There is a huge amount of artists copying that look.