YouTube uses multiple algos and signals, and yet spam still gets through. In some ways social media spamming techniques are reminiscent of the early days of the Web -- when the Web was overrun with "SEO" spam from junk sites attempting to manipulate the Google algo in the late 90s / early 00s. Google eventually solved the problem of Web and Gmail spam for the most part, but social media spam is a somewhat different beast.

HN has been having to deal with the in-authenticity problem too, maybe even moreso as of late. Solving it may require a concerted effort from all of us -- a vigilant watch by those who can see through -- those who not only take the time to flag inauthentic activity as they see it, but also educate others by calling it out and pulling back the curtain in a way that reveals its nature so more may become aware and learn to see it too. Discernment is a skill that can be learned when we are cognizant and vigilant and conscientious about paying attention.

They definitely use a mix, the visual hash is just an easy one to demonstrate and was clearly the primary one used at some point in the past judging from the prevalence of simply reversed/mirrored pirated uploads years ago.