What concept suggests that small bacteria were engulfed by larger cells to become mitochondria and help to self flourish and become a eukaryotic cell

Endosymbiotic theory

Explanation:

The concept that hypothesized that small bacteria were engulfed by larger cells to become the mitochondrion  and eventually become a eukaryotic cell is known as symbiogenesis or the endosymbiotic theory.

The endosymbiotic theory argues that organelles like mitochondrion and the chloroplast found in modern-day eukaryotic cells were formerly prokaryotic microbes. These microbes got ingested by primitive, amoeba-like eukaryotic cells that were first formed but instead of dying, the engulfed microbes found a way to symbiotically coexist with their hosts, eventually forming the organelles - aerobic microbes forming the mitochondrion and photosynthetic microbes formed the chloroplast.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: