Trading volume and market cap are 2 completely different things. A coin can have almost 0 volume but market cap in the millions if its very illiquid.
Yes, trading volume and marketcap are two different things, but you can know from marketcap of what coin trading volume can be (either high or low). The higher the marketcap, the likely higher the trading volume can be. I am not talking about marketcap in general, but of a particular exchange, comparing it with the trading volume of the exchange.
Volume is typically used when day trading because it gives you hints when reversals can happen. Usually during a pivot there is a big volume bar and it hints that it might reverse. Not 100% accurate but during certain times it works very well.
Yes, volume is used in trading, to make analyses. Even I have not seen an exchange with wn indicator for marketcap, not necessary and have no useful information , but trading volume is used as an indicator in trading.