From what I understood is that everything runs on public clouds. They tried Microsoft, Google, and Amazon. Sooo they should have enough experience by now.
Sourcing here is 2019 article about just one AWS contract. Apple also uses Google Cloud and Azure extensively, not just tryouts, they were one of Google Cloud's biggest customers. They are also building their own data centers. (TL;DR it's much more complicated than these comments would indicate at their face)
The contract was 1.5B over 5 years