On top of what was already said with respect to extremely low body fat (and an unhealthy amount of dehydration), many take nitric oxide to further dilate their veins. I would say that if you or I got super lean and worked arms a lot, we would have bicep veins but likely not in that huge squiggly shape that professional body builders have.
Veins are kinda weird though. They have a general outline that they tend to follow in everyone, but you can get random branches/tributaries all over the place.
Source: am a nurse who likes doing IVs
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