How do e-commerce analytics company work?

Hi. I am a soon to be CS freshman and I am a newbie in this

. I want to know how sites like Terapeak , JungleScout work?

I want to analyze a popular e-commerce website in my country(daraz.com.np if that matters ) and see the most sold product and see how all products are selling,etc. If anyone could point me in the right direction it will be greatly appreciated.

Thanks.