do all men wear underwear underneath their swimming trunks? ?

I wear underwear under my swimming shorts 

No, As others have commented swimming suits have a liner or supporter that prevents the cuck and balls from hanging out the from the leg.  As teenagers in the fifties we were concerned about erections and wore an jockstrap so that it would show less.  In fact at that time the coolest suits were reversible and did not have integral supporters.  

I don't understand why I see men wearing underpants with swimming suits.

Not really as their swimming trunks are like underwear.

But it also depends on the person if they want to wear underwear or not. 

My Swimming shorts have the underwear built-in

it saves the risk  of embarrassment

Oh Dam forgot the Period  

No and Yes to you questions. Some wear underwear underneath the swimsuit but that will slow you down when you swim.

I do not wear underwear underneath when I swim and I only wear swimsuit to swim in the pool so I do not slow down too much. 

You could choose to wear underwear underneath or not wear underwear underneath the swimsuits it is your choice. 

I think I misread your question. Don't you mean "Does any man, any where, for any particular reason that makes any sense, wear underwear underneath thier swimming trunks"

Since I don't wear anything when swimming, there really isn't anyplace to conciel such a disturbing choice of swimwear as underwear. Real men don't wear anything, anywhere, for any reason, because anyone who would is not a man.

If they want to!

I wear swim briefs and I wear a small compression thong under them to keep all my pieces down there tucked away so I won't have any bulge

No, most do not.

I don't ( and I don't wear anything at all😉)