Welcome to Microsoft Q&A, thank you for posting your question here!
Your question is how this "Azure cloud" decentralized, if it is all under one corporation's computers.
First of all, let's look to a basic definition of "decentralized" design. Decentralized simply means the design is controlled or can be controlled by several local offices or authorities rather than one single one.
So, Azure cloud "decentralized" design have many services such as IaaS, PaaS, SaaS which are flexible enough to use as a service and also allow you to manage any of these services. While Microsoft Partners or Managed Service Provider (MSP) or Microsoft corporation or "YOU" can also take control of.
You have your roles to play, the host have their roles to play. This makes sense in decentralization, and you can also take some percentages of ownership of your Azure Cloud Services.
Hope this helps!
Kindly let me know if the above helps or you need further assistance or explanation on this issue.
Best Regards,
Sina