What is the meaning of `struct X typedef` vs. `typedef struct X`?

The fact that both typedef <type> <alias> and <type> typedef <alias> are valid simply comes from the language grammar definition.

typedef is classified as a storage-class specfifier (just like static, auto), and the type itself is known as the type-specifier. From the syntax definitions in section 6.7 of the standard, you’ll see that these are free to be interchanged:

declaration:
    declaration-specifiers init-declarator-list ;

declaration-specifiers:
    storage-class-specifier declaration-specifiers
    type-specifier declaration-specifiers
    type-qualifier declaration-specifiers
    function-specifier declaration-specifiers

init-declarator-list:
    init-declarator
    init-declarator-list , init-declarator

init-declarator:
    declarator
    declarator = initializer

(Note, of course, that this is equally true for structs and for non-structs, meaning that double typedef trouble; is also valid.)