Benefits or effects of salt on fruit fruit cleanliness with regards to wax, pests, pesticides, insecticides, etc?
According to this article, submerging strawberries in saltwater will make fruitfly larva leave the berries. Apparently the idea was popularized in May 2020 by a TikTok post. But the author goes on to say that it's probably not necessary, that consuming fruitfly eggs or larvae is not harmful (they site USDA for this claim), and soaking your strawberries in saltwater may ruin the flavor. I get the impression that the author of this article didn't actually wash any strawberries in saltwater themselves (given that they only say &quot;may&quot; ruin the taste).
This article cites India's Centre for Environmental Science as recommending washing fruit in 2% saltwater to help remove pesticide residue from the fruit surface. They say that  most pesticide residue will be removed by washing with normal cold water, but saltwater removes even more pesticides. Here's an article from the Centre for Environmental Science, which has the information cited in the previous article.
Apparently there is also a myth that washing fruits and vegetables with salt water is a bad idea, that it will make pesticide residues &quot;more durable.&quot; That myth is debunked here. This article cites Dr. Jessada Denduangboripant of Chulalongkorn University, as saying “salt water can be used to wash fruit and vegetables” and &quot;Most insecticides come in oil form so salt will make the oil compound separate from each other.&quot;
Since point of the salt is to help remove oil-based pesticides, you might think of washing your produce with detergent. The problem with that idea is that detergents are not tested for food safety, and you risk leaving some detergent residue on the produce (or even of the produce absorbing the detergent) which you then eat. Eating detergent is not good for you. The advantage of salt is that if you don't manage to rinse it all off, the remaining traces of salt are non-toxic.
All the sources I found about this mentioned using saltwater, not dry salt. So I don't think the salt is meant as an abrasive. Maybe it would work for that, but I didn't find any references for it.

You mentioned a language barrier with the relative who told you about using salt to clean vegetables. Given that two of the top search results were from India and Thailand, and that I've never heard this technique recommended here in the US, I wonder if this might be a technique which is more common in southeast Asia, and perhaps originated there. Some of the recent sources referenced Covid, so perhaps the idea came about when everyone was (unnecessarily, as it turns out) trying to sterilize their vegetables in the early stages of the pandemic. Or perhaps the idea already existed, and it had a surge in popularity for the same reason. Pure speculation on my part.