late 2009 Imac Not booting into MacOS

i bought 2 iMac late 2009 both with no hdd. i put a SSD in one and hdd in the other. both iMacs refuse to boot into macOS just displaying a white screen with grey question mark folder. So i tried to boot of a usb with macOS. but the iMacs refuse to give the option to boot off the usb even when trying difrent key combinations form the apple website which don't open anything or do anything. But they both will boot onto a windows install media usb. any help would be great been stuck on this for a while.