Still have no idea

you are not

Nope

Most swim shorts come with "netting". This is pseudo underwear built in so that you don't feel so uncomfortable, and you don't flash anyone by accident. Some don't come with the netting as some people don't like it. For those that don't, the choice is yours whether to wear underwear or not. Speedos or similar type vestments are "not" designed to have anything between your skin and the material. However, if you wish to make a design statement and wear your boxers with your speedo feel free. Just know we are probably not laughing with you but at you.

You do you man, who cares

More room for sand to get stuck on your skin. Instant sand paper