I encourage everyone to read the "physiology" chapter at https://en.wikipedia.org/wiki/Adipose_tissue

TLDR: Blood sugar spikes above certain level are toxic, so the body responds with an emergency action of stashing the sugar wherever possible, including muscle, liver, and fat tissue. The former two are rather limited in their capacity, while the latter is unlimited. Ongoing elevated levels of sugars in the bloodstream is an ongoing cleanup job for the fat cells, and they never get aground to release the accumulated energy back into circulation.

At the "hardware" level, that's all there is to obesity - overabundance of quick carbs (sugars, flour, starch).