It's called value destruction. The coins value dropped very quickly, the counter parties then where either 1) Most likely nobody, (as in nobody cashed out, as the myriad altcoins approached the limit of worthless) and 2) traders on FTX who cashed out other ,"more valuable" coins they were lent after depositing altcoins as collateral.

They were the counterparty with themself?

They accept FTT as collateral, give out a leveraged loan to the other company. The company spends that money on things like stadium rights and bailing out other failed crypto companies. Collateral falls to a value of 0, and the money is spent.

The thing is that there was never any funds stored in these accounts. If you store a bitcoin then there isn't any value to it until you sell it.

The money that you spent to pay for the bitcoins exit the system during the purchase. Most likely paying the Ferrari of the seller.