How do Subtract, Weld, Intersect, and Exclude decide what to keep?

Layer order is the rule: the BOTTOM object in the selection is the base. Subtract cuts the upper shapes out of the base (like Illustrator's Minus Front); Weld fuses everything into one shape; Intersect keeps only the overlap; Exclude keeps everything except the overlap. Check the Layers panel before a boolean if the result surprises you.