What you're referring to is, I believe, proximal causes and distal causes.
Distal causes in depression's case maybe stuff like sleep deprivation, overstressed immune system, gut biome issues, etc. (and possibly genetic elements, who knows!) with little known about how they cause it or how they interact/overlap with one another.
The immediate proximal cause is, of course, the "chemical imbalance", which is popsci speak for neurotransmitter issues (among possibly others), a higher level proximal cause might be a life event that somehow causes some 'critical mass threshold' leading a spiraling avalanche (though again, little is known at the biological level about why this is so, assuming it is so.)
Distal causes in depression's case maybe stuff like sleep deprivation, overstressed immune system, gut biome issues, etc. (and possibly genetic elements, who knows!) with little known about how they cause it or how they interact/overlap with one another.
The immediate proximal cause is, of course, the "chemical imbalance", which is popsci speak for neurotransmitter issues (among possibly others), a higher level proximal cause might be a life event that somehow causes some 'critical mass threshold' leading a spiraling avalanche (though again, little is known at the biological level about why this is so, assuming it is so.)