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> gets us to about 150 trillion humans in 1,000 years

I realize the big issue is settled here but just to scope this detail: the total land area of earth is coincidentally approximately 150 million km^2

At 150 trillion humans each human would get just about a square meter. Many of those square meters are uninhabitable.

That’s about the density of Kowloon walled city. And ignores the available space in the ocean.

Did they grow any food, produce electricity, or had any manufacturing at all in Kowloon?

To be clear, "Kowloon" is different from "Kowloon Walled City". And, yes, they did have very small manufacturing inside.

I highly recommend that you find a copy of this (in)famous book about it: << City of Darkness>> (https://cityofdarkness.co.uk).