Sorry I can see how you find that confusing. Market makers "spread" their trading. If you buy bitcoin a market maker sells it to you, that market maker is now short bitcoin. To offset his position he goes and buys ETH (or other shitcoin). That makes him -1BTC +1ETH. To close out that position that market maker goes and sells the ETH-BTC spread. Selling that spread (-1ETH +1BTC) offsets the original -1BTC +1ETH spread. This makes him flat and he nets his dollars profit