Deep-water formation in the eastern Subpolar North Atlantic Ocean (eSPNA) and Nordic Seas is crucial for maintaining the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), of consequence for global climate. However, it is still uncertain which processes determine the deep-water formation and how much Atlantic and Arctic waters respectively contribute to the lower limb. To address this, here we used Lagrangian trajectories to diagnose a global eddy-resolving ocean model that agrees well with recent observations highlighting the eSPNA as a primary source of the AMOC lower limb. Comprised of 72% Atlantic waters and 28% Arctic waters, the density and depth of the AMOC lower limb is critically dependent on Atlantic-Arctic mixing, primarily in the vicinity of Denmark Strait. In contrast, Atlantic waters gaining density through air-sea interaction along the eastern periphery of Nordic Seas and not entering the Arctic Ocean make a negligible contribution to the lower limb.