As I said tou you
here, I don't think I'd use the term "Container" for Spring. Spring is a provisioner and a configurator, and it supplies a number of frameworks, but the actual "container" for a webapp is the deployed WAR in the webapp server.
And since Spring is not a standard
J2EE or JEE service provided by the container, that means that each webapp has to include Spring (or not) if it needs it as part of the WAR.
It's "loaded" as simply another set of classes the same way all classes are loaded. There's no Spring
Thread or process, just a Factory that the application can draw upon whenever it needs a particular bean.
And the answer to your third question was given in the other message thread I mentioned.