The exchange uses two wallets I guess because the currently active users have their hot wallet to make a received and make send for the users for their withdrawal, this must be active as always because the number of users always make a transaction, and the cold wallet is the reserved asset for the possible lack of the hot wallet I don't quite sure if they make a program at the same time that the cold wallet automatically refill a hot wallet once its already near to its limit but well its their algorithm, and also those known exchange are using different layer of security for their verification to preventing losing amount of money for each transactions.
Thinking with you, they use hot wallets to serve the deposit and withdrawal of users. Since there are not only bitcoins, but there are so many different tokens that hardware wallets hardly support, it is imperative that they use hot wallets to provide users in that case. They will use cold wallets to store larger assets and only use them when hot wallets lack liquidity. But I think the company's employees will do the asset transfer, they don't use algorithms or bots. Most major exchanges have a very large number of employees to serve many different stages, they operate like a traditional company.
