There is only one global object and it is created before control enters any execution context.
Unless specified otherwise, the standard built-in properties of global have PD{[[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}. So they don't show up in a for-in and can be altered.
It has no [[Construct]] or [[Call]] property.
The value of [[Prototype]] and [[Class]] are implementation-dependent.