Younger girls often wear underwear beneath their volleyball shorts but older girls and most professional players do not.
Volleyball shorts are meant to be worn as a one piece item with no panties needed. The shorts are a lot more comfy when worn without underwear. The shorts will feel closer and movement will be less restricted with nothing underneath.
If you have your period its important to use a tampon when wearing volleyball shorts. You don’t need underwear on your period either if you have your tampon properly sized and inserted.
Its common for girls to experience discharge when wearing volleyball shorts. The shorts are meant to absorb this wetness.