I frequently get questions about Bluemix architecture. Bluemix architecture is described in the documentation. But I feel this is not good enough to appreciate how things are going inside Bluemix. Good thing on Bluemix is that this is based on open technology called Cloud Foundry. And there are good materials are available on the internet.
Here is some.
Cloud Foundry | How it works
This is for beginner of Cloud Foundry users.
Cloud Foundry V2 | Intermediate Deep Dive
This is for users who just started to develop applications on Cloud Foundry base PaaS, especially who started to use Buildpacks.
Thanks to @jacopen, these were made available for public.
Both chart describes Cloud Foundry in more detail than Bluemix documentation. If you are fan of Cloud Foundry, there are worth to check.