Filler bots are incentivized to listen for events, and fill orders, because they earn profit whenever they fill an order. These filler bots have wallet addresses with private keys and public keys, as does the user, and SushiSwap. In order to execute the swap itself, the filler bots call the smart contract for the pair requested by the user, if/when the price reaches the value set in the limit order. The smart contract in turn will fill the order by sending funds to the users wallet, and in the process, it sends the limit order fee to the filler bot as profit. This exchange of funds is essentially an exchange of balances, that is communicated via an encryption. The magic of public key cryptography allows for these messages between the users’ wallets, the filler bots wallet, and SushiSwaps smart contracts to pass securely, because the private keys of these entities are obscured by cryptographic algorithms that can only be solved in one direction . The messages passed between them can only fill the order to the parameters initialized by the user, because decrypting them requires having access to a private key, which is only visible to the entity who initialized the order (the user).