Rails' respond_to - syntax analysis - ruby-on-rails

Question

Here is a Rails code:
respond_to do |format|
format.html
format.xml { render :xml => #users }
format.json { render :json => #users }
end
I know what it does. But I don't know the meaning of the commands syntax-wise.
format.xml -- what is xml, is this a method which an object format has, correct? Where do I find its signature (or description)?
{ } -- a block or a hash? I think this is a block.
render -- a method? where do I find its signature (where in api docs)?
:xml => #users -- a hash, where :xml => is a key, correct?
So it could be reprented as, right?:
respond_to do |format|
format.html
format.xml do
render(:xml => #users)
end
format.json do
render(:json => #users)
end
end

Answer 1

format.*
Using the debugger, I learnt that format is an instance of ActionController::MimeResponds::Collector. It includes AbstractController::Collector, which uses method_missing to respond to various format calls.
if Mime::SET.include?(mime_constant)
AbstractController::Collector.generate_method_for_mime(mime_constant)
send(symbol, &block)
else
# ...
{ render xml: ... }
Looking at method_missing, it's clear that it expects a block.
render
Yes, render is a method. Documentation.
:xml => #user
Yes, it's a hash. The key is the symbol :xml. It can also be written as
render xml: #user
render(xml: #user)
render({xml: #user})
render({:xml => #user})

Answer 2

format comes from the request made to the method, http://app.com/controller/method.format where format could be .html or .csv etc. The meaning of format.xml is simply to check if the user requested an xml page (http://app.com/controller/method.xml.
Block, just like you describe.
At the apidock! http://apidock.com/rails/ActionController/Base/render
Correct, render will act depending on the key that exists. In your case, as the existing key is :xml the render method will output #users formatted as XML.