From what I understand it’s actually very simple. Imagine a multisig wallet with all of us part of one giant wallet. So with zaps the Satoshis are just moving (ledgering) within one giant wallet among users. So it’s free. Anytime you send tx-info to be entered into a block, you pay tx fee. Not sure if I answered your question.
By opening channel (from your own lightning node) with eg. WOS, you just add a tube to that giant pot of a wallet. Not very clear how much liquidity is required before WOS let’s you open that channel.
Not sure that they would be willing either.