Booking.com was the world's third largest eCommerce site in 2018, when I worked at the company headquarters in Amsterdam, Netherlands.
Unfortunately, I'm not at liberty to disclose the details of this work. However, what I can say is that I was the lead designer on two teams:
The combined impact of the A/B tested website iterations of these 6-10 person teams was calculated to be worth tens of millions of Euros per year to the company. Each A/B test collected data for about a month, and then we reviewed the usage metrics and made the decision whether to go live with each change after seeing exactly by how much we'd improve conversion, or how we had impacted other key metrics like the number customer support tickets issued, or abandonment rates.
For a publicly accessible project during my time at Booking, see Designing the Payment UX at Booking.com.