DEX Screener uses the following formula to calculate fully diluted valuation for any given token:

FDV = (total supply - burned supply) * price

In most cases, FDV will be the same as market cap, but there can be exceptions. For tokens that don't burn supply that isn't in circulation, DS will look for self-reported circulating supply from Enhanced Token Info or CoinGecko, and automatically use that amount to calculate market cap instead.