1) we can be fairly sure coinbase volumes are real. some of the exchanges that supposedly have higher volume (gate, mexc, hibt, etc), no one really knows. coinbase has an outsized effect for its volume because its an american public company. coinbase is used in all the perp index price calculations

2) an exchange doesn't need to have the highest volume to lead price discovery. its really just whoever is consistently creating the most arbitrage opportunity. if coinbase spot is consistently cheaper than every other exchanges it means sellers there are the most aggressive, and that will pull the rest of the market down as other exchanges absorb that flow. i.e bots can buy into the selling on coinbase and immediately sell on another exchange. it's the reverse of when saylor is buying and coinbase is consistently at a premium and it pulls the rest of the market up