It's not a "pot of money" because the market cap is not determined by the amount of government currency traded for bitcoin. Trading $100,000,000 for 5,000 bitcoin does not increase the market cap by $100,000,000.
Similarly, trading 5,000 bitcoin for $100,000,000 does not decrease the market cap by $100,000,000.
Since there's a seller for every buyer, there's zero money going into any "pot".
The market cap is determined by what price buyers and sellers agree on at any point in time. That means the market cap can double, or halve, instantly, even though trading volume is a tiny fraction of the change in market cap.
There is no pot.
And it works the same way for every other market, from stocks to bonds to gold and real estate.