Always check the pairings on each exchange. Most coins don’t pair directly with other coins. BTC pairs with everything. ETH pairs with a lot of coins. USDT pairs with a lot of coins. There are a few other exceptions on certain exchages but other than that you generally need one of the 3 I just mentioned as the go between. Always check the coin you want to trade for on CoinMarketCap and click the Markets tab below the chart to see all the available pairings for that coin.