If I give you 10 dollars, and then you put it in your pocket, and then you take it out again and give me 10 dollars back - no actual economic growth occurred. It's simply shuffling money around; the amount stays the same.
Two economists are walking down a forest path and they come across a piece of animal excrement. The first economist turns to the second and says "I'll give you $100 to eat that!" The second one eats it and the first one gives him $100. They start walking again and a few minutes later they come to another piece of animal excrement. The second economist now looks at the first and says "I'll give you $100 to eat!" The first one eats it and collects his money.
They walk a bit more and the first one says, "You know, I gave you $100 to eat shit, and you gave me the same $100 to eat shit. I can't help but think we both just ate shit for nothing." The second one responds, "That's not true at all! We increased the GDP by $200!"
It's not an argument, it's a joke. And the crux of why it's not a real argument is if you're willing to do Job X for $N, there is basically no scenario in which you're also willing to pay $N out of your own pocket for someone else to do Job X.
But it’s not just dollars changing hands. If I invest N dollars in your hot-dog business, and you use some of that capital to buy equipment from me, I get revenue from the sale, and I still own an investment in your business. If you succeed, that investment can also make me money.
If I loan you 10 dollars at 50% interest rate and then you use the 10 dollars to buy a product I am selling and you proceed to make $11 billion dollars with that product, you just made $11 billion dollars from $10 investment and I made $5 - not bad
