I can't say for certain but on Twitter there are lots of accounts that follow the daily flows and they report down to the last Bitcoin how much is bought and sold. As far as I know only one ETF publishes their address so anyone can see how much Bitcoin they hold so that should provide some proof but I have no idea about all the others. Here is an example of one of those posts.I recently watched a video that said the ETFs don’t actually have to buy BTC immediately when you purchase a share of the ETF. It said they have like 30 days or something like that. Is that true?
