They also don't take a fee for usage. The component that takes a fee is the Relay network, which simply forwards withdrawals to wallets that don't yet have any

They also don't take a fee for usage. The component that takes a fee is the Relay network, which simply forwards withdrawals to wallets that don't yet have any currency (and therefore cannot pay the transaction cost of withdrawal itself), and take small fee for the service. They're just gas forwarders, not anything really special to Tornado Cash's function. But you can use the Tornado Cash pools themselves without the relayers.