בראשית מג:כח פירוש: רש״י, ספורנו, רד״ק וטור הארוך

ויקדו וישתחו AND THEY BOWED THE HEAD AND PROSTRATED THEMSELVES — in recognition of his enquiry regarding their welfare. The root קדד denotes bowing the head: the verb שחה in the Hiphil denotes prostration upon the ground (Megillah 22b).

שלום לעבדך לאבינו עודנו חי, also your servant our father is well. He does not enjoy the serenity of the dead but he is still very much alive.

ויקדו וישתחוו, when they prostrated themselves this was with the intention of making an obeisance to their father after whom Joseph had inquired (although outwardly, this was not noticeable).