You can't swap ETH for BTC on a decentralized exchange because those two crypto-currencies are on two different blockchains. Metamask swap uses multiple decentralized exchanges. What you can have by using the swap function of Metamask is WrappedBTC which is an ERC20 token located on Ethereum.
What you can do is directly send your ETH to a centralized exchange (as Binance). There is a "deposit" part. You will then have an Ethereum address. From your Metamask, you will have to send eth to this address.