From what I read, someone who sells btc sometimes has trouble receiving the fiats in its bank account. If the sum is large, the bank turns on all its red lights. I guess it's not a problem for someone famous, but for an ordinary citizen, how it happens if he lives modestly, recovers his private keys that he lost 10 years ago and finds himself with millions?
Offshore banks and offshore companies are a thing. Have you asked yourself why trillions of dollars are being kept in offshore companies located at "tax heaven" islands? The rich people are using such companies because they are very convenient.
Selling big amounts of BTC in centralized exchanges seems very risky to me. I guess that the crypto whales are negotiating big deals with the crypto exchange owners and they sell them their BTC outside of the crypto exchange platforms.
Obviously we are not crypto whale accountants and we don't know how the crypto whales are moving their wealth, so I'm just guessing.