In aggregate, capitalists make what they spend and workers spend what they make. If you build a house, you spend a bunch of money from your savings on concrete, lumber, pipes, wires, and labor, and you end up with a house that is worth, you hope, more than you paid to build it. The money has left your possession but a new thing, a house, worth that amount is now in your possession. The money is out there somewhere, and the house now exists too. A profit is created through money invested in creating a valuable object because the total value increased. However money is not a fixed amount. It expands and contracts because banks can create the funding to buy the house by changing numbers in a computer and paying interest on the resulting deposit. The bank will create a bank deposit (a mortgage) for someone to buy the house from you. So the amount of money is generally expanding and collateralizing the real wealth (the house). That way further profit is created through someone spending from their (future) savings. People can only spend so much from their savings and then spend so much from their future savings though, so the real wealth needs to increase more than debt and prices need to make sense or you get a financial catastrophe.