Hey so the Relayer and bundler are two different but similar things.
relayerUrl refers to biconomy’s relayer node, which is responsible for signing all kind of valid transactions, paying for their gas fees and sending them to the network.
bundlerUrl refers to a bundler running on network, can be anyone’s bundler. The task for bundler is to send valid EIP-4337 Account abstraction transactions. Bundler receive’s multiple UserOps, bundles them to call handleOp on entry point contract. They get paid for doing so directly via paymasters.