When I was doing affiliate marketing a decade ago, what I did manually was watch informercials and jot down the names of all those products. Then manually run searches in Google Trends to see which ones were spiking, and create content for those products (ie reviews, guides, etc). Here's an example of a product that really took off after they did an informercial in 2006: https://trends.google.com/trends/explore?date=all&q=almighty... . I created a page for this product and was ranked #1 instantly. Because nobody else was writing about it.

I stopped doing affiliate marketing so don't know what people do now, but just throwing you some ideas.