Xinhua indexes 且 under the 一 radical, picking the bottom horizontal as the anchor stroke. Reading the picture: the offering stand resting on level ground — a single base line that grounds the stacked frame above.
The upper four-stroke frame originally pictured an ancestral tablet or stacked offering stand, not the moon despite the modern shape match. It sits on the base line below to form the full ritual-stand silhouette from which the function-word meanings 'moreover, and also' later branched.
