Some swimming trunks have built in underwear but they’re tiny like briefs
I wear normal trunks or briefs under my swimming trunks. Bf currently has one with built-in underwear, but his old one was like mine, and he just let his slong bloat lmaoo
I used to wear skirt swimsuit it's come with shots attached so I also wear a regular under wear with it
Admittedly, I'm old, but it never once occurred to me to wear regular underwear under my swim shorts since every pair I've ever had includes mesh briefs sewn in. I HAVE been in the water w/ normal shorts and undies though and it's not pleasant. the fabric is much more absorbent and prone to chaffing-- especially once you've gotten out if you don't get changed immediately. Though I imagine that's the same for people w/o wieners too. All that damp cotton rubbing up on your nethers isn't good for anyone.
it occurs to me that maybe the people you've seen swimming w/ the waistband of their undies peaking out may have been cheap-ass non-thinker types that don't own a swim suit or don't want one for whatever reason... so they use nylon shorts that are more-or-less the same thing but they don't want to go commando for Free Willy reasons so they also wear their tighty whities.
OH! OH! I have a couple of follow up questions to @GrandDetective topic if some guys want to answer! First of all, does it feel uncomfortable to swim with your underwear and swimsuit on? I often see guys with swimsuits and the waistband of their underwear peaking out, so I often wondered if it's bothersome to yall. Also, do you wear a specific tipe of underwear or just whatever you have on works? Idk if it makes sense, but it's such a curious topic to me. If you have ANY fun fact about all of this PLEASE write it down, I'm SUPER curios!