Ik this is wierd

Ik this is weird but I've only had girls and my friends that let me watch the gender reveal ultrasounds were all girls as well but how different is the boys on the ultrasounds like on a girl u see its clearly a vagina but i wanna know how to tell if its a boy and not just the cord between thier legs if someone would like to show off a pic I'd really appreciate it. I feel clueless on boys