So the swap program acts like a trustless intermediary, essentially making temporary escrow wallets for both the buyer and seller to verify both parties have the funds before releasing.

In this case, the seller deposits a certain amount of XMR into a new, temporary XMR wallet created by the swap program. The seller then advertises to buyers they are selling XMR at a certain rate. OP, the buyer, sees the advertised rate, deems it acceptable, and sends funds to a new temporary BTC wallet created by the swap program.

After a certain number of confirmations, the swap program determines the funds have been deposited by the buyer. Verifies both parties have deposited sums at an agreed upon rate, and releases the XMR to the buyer and the BTC to the seller.