Key fact (left out of the article): most of the mitochondria's essential proteins, including those for ATP/energy synthesis, are produced by the host cell from

Key fact (left out of the article): most of the mitochondria's essential proteins, including those for ATP/energy synthesis, are produced by the host cell from the host cell's DNA. So yes, mitochondria have some DNA and do replicate, but no, their cell is not just an "environment" for them. Also interesting: mitochondria can join (often to rescue one failing due to transcription errors) and be transported to other cells across bridges (to supercharge the recipient, as they want to do now for immune cell therapy).