ELI5 Weight Gain - how does the body decide where it goes?

How does the body decide where to add or take away fat when gaining or losing weight? Why is it that lower stomach fat and lower back fat are harder to lose compared to other areas? Why does working a muscle with low weight and high volume vs high weight and low volume yield different results? Why do powerlifters wear those big belts around their waists?
... we explain like I'm five
Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: gailson0192, sirduke6, limbonics, maxwellvador, maximov86, dirtydbag, Mookie_Merkk, djmcmayhem, thetreece and bakemechocolatecake
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