O365 workloads are distributed across a number of environments. While Exchange mostly lives one its own hardware, SPO is very distributed now - it still has its own hardware, but also has a lot of data and services that run in Azure. And O365 is very dependent on AAD.
Bits and bobs run on top of pure azure, and some infra is definitely shared, but anything cloudy that came up around the same time as or before azure (like Bing and o365) used and for the most part still uses their own data centers and deployment tech.