In the early days, they had a ton of special configurations, like w.amazon.com - the internal wiki, which was probably running in a datacenter somewhere.

But in recent years, they run entirely on AWS. They even have solutions architects to help them build things in a resilient way. They also spent years ripping out Oracle and replacing it with RDS MySQL, PostgreSQL and Aurora.

It is true that Amazon runs on AWS.

Source: worked there.