What is the theory of endosymbiosis, and why are we discussing this in a unit about energy generation in microorganisms?

The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. ... They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.

this would be a math

the cost of mining is extremely low compared to the money earned from mining, creating income to improve the environment.