Each customer's funds aren't keep in individual wallets. It's pooled into a wallet controlled by the exchange.
Exchanges do put most of the assets in a cold storage. Binance has $155B in assets and handles $14B of trade volume in 24 hours so not all of it are in a cold storage.
Exchanges do put most of the assets in a cold storage. Binance has $155B in assets and handles $14B of trade volume in 24 hours so not all of it are in a cold storage.
https://coinmarketcap.com/exchanges/binance/