Testosterone is directly causally inverse to bodyfat in men (once above some very low baseline)

Fat directly converts testosterone to Estrogen via a process called aromatization.

Personally my Testosterone close to doubled when going from 25% bodyfat to 13%. I get blood tests regularly and can see the levels fluctuate pretty closely with fat levels

In the past decade, research suggests that testosterone levels have actually gone up[1].

[1]https://www.cremieux.xyz/p/why-are-testosterone-levels-risin...

Really makes the study completely pointless when the last 50 years has also seen the rise of the global obesity epidemic.