Could be either most bought, most searched or most viewed or any function of these imo.

You’re asking about “recommendation algorithms” if you want to do some more formal reading on the topic.

The specifics of how they function are often among a particular company’s more closely-guarded trade secrets. TikTok, for instance, is famous for the efficacy of it’s. Google is pretty good at it too, and while I haven’t used his properties in awhile, I suspect Zuck’s is also perfectly competent at doing what he wants it to do.

Reddit’s was always pretty weak, which I actually liked, as the content was more random instead of being more carefully selected.

I have worked with a handful of online stores, all of them used different criteria for what was the most popular item. Some were pretty simple like “highest volume of sales in the past 7 or 30 days”, some others were pretty complex and some were pretty much dishonest, putting expensive products next to slightly higher than average ones in order to make them feel cheaper. Like the other comment have said, there is no consensus and it mainly depends on the company’s goals with that section.

Highest profit margin