Believe it or not but it's true... I invite you to watch making of, behind the scenes & interviews, the team already said that it's prosthetics. Famous or not, it's a fiction, not real life or porn, even if the show is super reallist and far from disney or netflix perfect world.
The people from the makeup department who make the penises have been interviewed before about this.
If the penis is erect, it's fake.
If the penis has to do something other than just be present, it's fake. (Liquid involved/Cal Peeing.)
If there's something "different" about the penis (ie micro, huge) it's fake.
If the actor is uncomfortable showing their real penis, it's fake.
If the actor is comfortable and the penis is just "there" and doesn't have to do anything, (ie. the actor is just standing there or walking through) then it's the actor's penis. It's cheaper (they cost between $6,500-$9,000 a penis). It saves time. So I doubt anyone is forcing one on someone in a locker scene where we only see them for a few seconds if the actor is okay with it.
In other words, not every penis is fake on the show. Many are but not all.
The "toilet" guy for instance, flat out said his was actually his own. Barbie Ferreira said the locker room penises weren't fake although I think at least one was. It would have cost thousands to shoot that scene with multiple fake dicks.