The client library deploys the smart contract on setup from a quick glance - here is the compiled byte code of this contract in the library:

https://github.com/toncenter/tonweb/blob/master/src/contract/payments/PaymentChannel.js#L41

Another interesting thing to know is that the entire logic of payment channels is just in this smart contract and the client library. I don’t believe TON node core was changed to add any special support. This feature could have been added to TON by any third party