Official Answer: it's an implementation detail and you don't need to know.
It is of course knowable for a specific (current) version of the framework:
1) if this is inside a method, then mystring
is a reference variable, usually on the stack. But when it is captured, or when the method is async, or ... it is moved to the heap.
2) if this is inside a class, then each instance will contain a mystring
reference as a field, stored on the heap.
In both cases there is an anonymous string object on the heap containing "Hello World"
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