In this Openstack Architecture, by Russell Bryant presentation from Redhat 2013 summit we can see description of all Openstack components together with best practices how to deploy them to achieve maximum scalability. If you are interested in more details this link will help you to take the next level: Openstack software architecture.
The amount and verbosity of available information can be overwhelming and it may become difficult after reading all of it to answer a single question:
How does a single VM is created in Openstack and how does the Openstack systems interact together to achieve it.
The slide below shows the six steps and hides the necessary complexity (at least at the beginning, for another view take a look at Event flow when a cloud instance is provisioned in Openstack).