I think that's where you're confused: It's not two transfers that require your interaction. A swap out is a single transaction as far as you can be concerned. You enter a bitcoin address and then you will be presented a lightning invoice. When you pay this lightning invoice, you will pay via lightning but you will receive sats into the onchain address you entered since they send a part of their sats that are already onchain to you.

I would recommend going to https://boltz.exchange/ and trying it out. You don't have to actually pay the lightning invoice at the end to see how the flow is.