Neoprene shorts

What do people wear under their neoprene shorts? I’ve just been wearing my normal underwear but by the time I’m finishing working out they’re usually drenched with sweat and in the past week or so (might be unrelated might not) I’ve developed a nasty rash in certain places and I’m not sure if it’s because of the heat and sweat situation created by the shorts? Do people wear nothing underneath them? Or like an under short sorta thing?