It's a common joke that Ruby is designed to make it as hard as possible for parsers.

For the code samples going forward, keep in mind that in ruby you don't need a literal return statement to return a value from a method. Just whatever value is last, is returned:

    def val
      5
    end

That method would always return 5.

> But then if the b() call itself returns a function, then is that function auto-called as well?

No, the only way to pass a method is by using method(method_name_here), which returns a Method object. To execute a Method object, you need to call call on it:

     def a
     end
     method(:a).call

So if you have a method that returns a method, it should not be automatically called, so there's no loop necessary:

     def b
       method(:a)
     end

     z = b

Then z is a Method object, namely a.

Some experiments to answer your questions (I admit I wasn't sure for all of them what the outcome would be). IRB is Ruby's repl:

    irb(main):001:0> def a
    irb(main):002:1> 5
    irb(main):003:1> end
    
    irb(main):019:0> {a: 1}
    => {:a=>1}
    irb(main):020:0> {a(): 1}
    SyntaxError: (irb):20: syntax error, unexpected ':', expecting tASSOC



    irb(main):009:0> def h(num = 10)
    irb(main):010:1> num
    irb(main):011:1> end

    irb(main):012:0> h
    => 10
    irb(main):013:0> h-3
    => 7
    irb(main):014:0> h -3
    => -3
    irb(main):015:0> h - 3
    => 7

    irb(main):001:0> a?b:c
    NameError: undefined local variable or method `c' for main:Object