BFI - landing to the north

Could someone tell me how large planes, like cargo, coming from the north, say Anchorage, are routed to final for KBFI landing north? The Jawbone approach simply says at Alki to turn to 160 heading and await vectors.Do the flights head south to the west of KSEA, then get threaded across that traffic coming into Sea Tac (also north landing)? It would seem safer to route them over Lake Washington on the downwind leg, avoiding the stream into KSEA.Thanks!Colin