How does Phusion Passenger reuse threads and processes?

Question

I am setting up an Apache2 webserver running multiple Ruby on Rails web applications with Phusion Passenger. I know that Passenger spawns Ruby processes for handling requests. I have the following questions:

• If more than one request has to be handled at the same time, will Passenger spawn multiple processes or multiple (Ruby) threads? How do I configure it so it always spawns single-threaded processes?
• If I have two Rails applications, imagine that a request for app A goes to process 1, then later request for app B arrives. Is it possible that process 1 will handle this request as well? When and how is this possible? In other words, is one process allowed to handle requests for multiple Rails applications?
• I have the same Rails application exported in multiple URLs and multiple virtual hosts (such as http:// and https://). Will the same process be able to serve different virtual hosts? (The answer to this seems to be yes, I've set a global variable in answering a request to virtual host A, and I was able to retrieve the value in virtual host B.)