Exchanges use liquidity enablers behind the scenes to move tokens around for large transfers. Examples include Fireblocks (virtual pool of liquidity) and Copper (actual pool of liquidity in cold storage using a derivative) which are both using MPC. If the exchange needs tokens quickly because of a large withdrawal, they turn to these suppliers which already have backend integrations. If user A withdraws from Exchange A requiring them to withdraw from a liquidity enabler to transfer to Exchange B only have Exchange B re-deposit those funds back into the liquidity enabler minus the on-chain fees and delays which took place to do the transfer on chain.
The FIO Protocol can enable exchanges to transfer funds between themselves without ever having to do the transfer on chain. It can do this because a FIO Request initiated by a user on one exchange is cryptographically signed by that exchange uses a private domain which only that exchange can create addresses on.
We need to model the business op according to the number of transactions per inter exchange withdraws and deposits. Also, we need to consider for popular assets (e.g. BTC, ETH).
Lastly, it would be worthwhile to consider how we model settlement and reconciliation to aggregate tier2 exchanges and wallets (with FIO send).