Internally, once the BTC is received, RUNE moves from the BTC pool to the ETH Pool, therefore it is a double swap (BTC:RUNE, RUNE:ETH). Inbound gas is paid in BTC, Outbound Fee is paid in ETH.
Swaps on THORChain are made possible by liquidity pools. These are pools of assets deposited by Liquidity providers, where each pool consists of 1 connected asset, for example Bitcoin, and THORChain's own asset, RUNE
Discover, find, and route swaps with the best rates with the exchange aggregator route finder. Split between different protocols and even different market depths all within one transaction in the THORWallet DEX mobile and webapp.