Population and upkeep
Population is the crop the buildings eat. Troop upkeep is extra crop on top. The number in the header is production minus both.
Buildings
Each upgrade adds population — crop the buildings eat. That is why a “finished” village still starves if you drop a Great Warehouse without more cropland.
Troops
Each unit has an hourly crop cost. A hero is 6. Roman cavalry can drop by 1 once the Horse Drinking Trough is high enough (Equites Legati 10, Equites Imperatoris 15, Equites Caesaris 20). Wonder villages halve troop crop.
Why this bites harder here
Upkeep runs between play windows. A train queue you started near the end of a session will keep eating at 02:00. If the granary hits zero, troops starve in the usual Travian order. Leave a surplus large enough for the whole gap between sessions, not just the next ten minutes.