When is node query called? Node resolver should be implemented on server?

Question

Background

I read Which relay objects must implement Node? page.

I could not understand about "diffing" process.

Questions

1. Does Relay use node query instead of a query designated by developers? In other words, Relay can automatically optimize or minimize queries by using node?
2. If any one of GraphQL Objects implements Node, Is it required that schema Query should have node and GraphQL Server should have node resolver? What is happen even if there are not implemented?
3. How to demonstrate these situations? Does anyone have examples?