Charles Ellicott Commentary Exodus 25:19

"And make one cherub at the one end, and one cherub at the other end: of one piece with the mercy-seat shall ye make the cherubim on the two ends thereof." — Exodus 25:19 (ASV)

Of the mercy seat you shall make the cherubims. —The meaning seems to be that the cherubim were not to be detached images, made separately and then fastened to the mercy seat, but to be formed from the same mass of gold as the mercy seat, and thus to be part and parcel of it.