It's a line above the nub usually at the end/tip, I think the first line is the scrotal bump whilst the top line is the developing penis. Sometimes it images at more than one line above the nub going down in sizes iykwim.
I'm not very good at explaining so I'll add a link below, hopefully it works
https://babynub.club/index.php?/topic/2707-click-here-nub-theory-and-more-explained/