They get a short soak (maybe 3 minutes) and roughly swished around by hand,
The reason I don't wash them is that mushrooms are sponges; they soak up liquid fast. When you fry them up , they're going to take a whole lot longer to brown. Yep, they'll brown, no worries, eventually.
Second thing is that mushrooms used to be grown in horse, or cow dung.
("Mushrooms on toast, darling"?
"MOOOOOOOHHH")
These days, the medium is totally sterile, so no health issues.
I don't buy mushrooms over here any more, because they're $22 a kilo. Yes, a small 400gm packet costs $9. In the UK, the same size costs $1.50 - $2, and if you are lucky, they're reduced to half price.
When I cook them, I start with garlic, olive oil, and nothing else. Once the mushrooms start leaching liquid, that's when I add salt.