for me, it is hard to believe or even accept that a brokerage can allow a client to borrow shares from them or from the market which the brokerage themselves don't even have. the only reason the shorts are being afforded the opportunity to borrow and sell and pay the massive interest rate is because they have margin accounts and have sufficient cash reserves to foot the interest charge and will be able to cover even if the price quadruples/quintuples. The brokerages know that these shorts will be "good for it".
But, even if they didn't have the shares to lend to their client, but another brokerage did, an electronic transfer could be done in a split second and paid for on the spot. If a brokerage is lending out shares to be shorted, they better be able to OWN the shares they are lending out at the price they are lending them out.
But, if I'm wrong, the fintel short shares webpage: https://fintel.io/ss/us/cydy
says that the available short shares number is updated every 30 minutes:
Short Shares Availability
This table shows the number of shares of US:CYDY available to be shorted at a leading prime brokerage. It is not the total number of shares available to short, nor is it the short interest.
It does not include data from other brokers or dark pools. It is a small sample, and it is useful for tracking the rise and fall in demand for shares throughout the day and weeks. It should not be treated as an absolute number of shares that are available to short in the market.
We update our database every 30 minutes but only display changes, in order to improve readability.