The documentary Made You Look: A True Story About Fake Art (2020) explains the concept and process better than I could hope to do myself, so to do your question justice, I would advise you to seek it out and to watch it. Exit Through the Gift Shop (2010) is adjacent to the topics raised and is also worth a watch.
https://en.wikipedia.org/wiki/Made_You_Look:_A_True_Story_Ab...
https://en.wikipedia.org/wiki/Exit_Through_the_Gift_Shop
Freeport facilities are part and parcel to enabling these complex businesses arrangements, and they are mentioned in an aside in the documentary Made You Look; the Geneva Freeport being one of, of not the, world’s oldest and most important such facility is called out specifically if I remember correctly, or maybe one in Antwerp.