Can people really see how many sales I get with my products?

My cousin just told me other sellers can see how much my product is selling. I thought all of my sales and volume were private? Does this mean just anyone can see my product successes and choose to copy-cat?

I’ve been in manufacturing long enough that I can take a pretty solid guesstimate on how much an item costs for manufacturing and packaging. But now can anyone just look at the volume*price and see where the cash cows are?

Googling says there’s sellers features but it seems surreal and when I google the exact OP question I get no info.