Whales sell OTC, they have brokers or services that buy and sell large amounts. This is all mostly happening off exchanges. An estimate 60-80% of trades are done like this.
It's exactly this. They're not going to pay 0.1% ~ 0.2% commission on a very large order to an exchange when they can deal person to person or pay a fixed fee to a dealer.
The whales you see on the exchanges are day trading not accumulating a BTC holding.