There are different levels of counterfeits, still most are obvious to the naked eye in person. Most are copied from ads so the colour is wrong, text is poor, wrong beat rate on the movement, etc. Poorly plated gold where it should be solid. On it goes.
It is actually not as hard as she makes it out for these but it is near impossible with the naked eye, with a loop things pop out fast. Modern Rolex will have Rolex engraved over and over inside the bezel plus the SN, Rolex laser engraves it, counterfeits (they are not fake or replica, they are counterfeit) will have this mechanically engraved. The print on the dial will usually be coarser on a counterfeit, same in her video. This is not one, but on a Submariner the gap between the bezel and the body is a tell tale sign. For the average seller the scum bags that buy, wear and sell them even have counterfeits now with full boxes and papers. Someone can buy a pretty nice legit watch for what these sell for...
It is a real greasy scum bag thing, and 100% reflects on the people who particpate in any way. Why, well it is obvious on the surface and of course blatantly illegal. Going deeper people pass away and the family sells (or tries to) their counterfeit watch and either end up ripping someone off or finding out that person they loved was a lier or a rube. It also causes issues in the grey market, same problems. There were even problems at Costco when they were selling grey market watches, scammers buy it, swap out the watch with a counterfeit and then return it (kid that takes the return has no idea), then it gets sold to someone that has no idea! In the end it hurts many people as it diminishes values, it is scamming, full stop.
Rolex is doing certified pre-owned now, but this comes at a premium. Ebay has a service where the watch can be shipped to them (their agent) and they verify it. I think we will see some form of NFT or like used in the future for authentication.
In the hobby and industry, there is a saying buy the seller not the watch. Also too good to be true....it is not true.