Is it documented, how the individual ETFs are holding their assets? I know they state that they do it via a custodian and name it like "Coinbase", but is that

Is it documented, how the individual ETFs are holding their assets? I know they state that they do it via a custodian and name it like "Coinbase", but is that really all? All they got is an IOU from Coinbase? Maybe they have more specific setups?