What is this 'pie'? The analogy seems to imply that there's some pre-existing pool of wealth from which people draw shares of varying size, and I'm fairly sure

What is this 'pie'? The analogy seems to imply that there's some pre-existing pool of wealth from which people draw shares of varying size, and I'm fairly sure that nothing like this exists in reality. Wealth doesn't originate in some pre-defined aggregation; it's generated by particular people in particular contexts. This concept of different people having different 'shares' of wealth relies on starting with some universal aggregation, and this always seems to be measured by calculating everyone's separate individual income and adding each of these values to into a single sum. But this implicitly acknowledges that wealth doesn't originate in an aggregation, so what's the justification for construing any aggregation at all?