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 

 This helps, thanks