These bitcoins that they hold in the fund are not theirs. You can go to your online brokerage, buy 1 BTC worth thru IBIT with your money and they will hold it for you. They cannot just sell it without your permission. Doesn’t work that way.
these bitcoin THEY HOLD are not YOURS when you are a end-investor of a broker
(not your key not your coin)
lets take this hierarchy
coinbase (custodian)
blackrock (etf trust)
robinhood (AP/agent/broker)
you (robinhood customer buying blackrock shares)
you as a share purchaser(end-user investor of an AP (robinhood)) become contracted to a share of a trusts collateral. where YOUR shares are held within robinhood assigned to you, which you cannot just ask for your one share to be converted to bitcoin. you have no claims on any btc
your share just represents a unit thats pegged to bitcoins PRICE, not ownership
you do not even get voting rights to tell blackrock to dissolve the shares and unlock btc
brokers(AP) (EG robinhood)
part of the basket share contract that robinhood(AP) have with blackrock, is where robinhood need to buy-back enough shares from its end users/have unsold shares to reach a "basket" amount of unassigned shares, to then have a voting right over that basket of shares to ask/get permission from blackrock to dissolve the shares and where blackrock(ETF) requests to coinbase(the custodian) to unlock the coins out of the ETF ownership. where the custodian then sells the coin to market to then give blackrock fiat who then give fiat to robinhood