When is the structure (user defined data type) created?

for example consider:

struct strct
{
 data member_1;
 data member_2;
 ......
};

when does the compiler recognize

struct strct 

as a data type? Is it after executing the line

struct strct

? Or after encountering the closing brace of the structure definition?

Declarations aren't "executed".

After reading

struct strct {

the compiler recognizes struct strct as an incomplete type, which is a type it doesn't know the size of. As you can use pointers to incomplete types, this enables you to write something like this:

struct strct {
    struct strct *next; // <- allowed, a pointer doesn't need the size of the object pointed to
    int foo;
};

Once the "body" of the struct declaration is finished, struct strct is a complete type, so you can declare variables of that type (the size must be known for that).

---

Side note: you could actually stop after the tag with your declaration like this:

struct strct;

and, as a consequence, have the compiler know an incomplete type struct strct. This is also called a forward declaration. Of course, it only makes sense when you have the complete declaration somewhere (possibly private in a module) as well. This is used for information hiding when implementing OOP code in C. You'd for example just declare something like this publicly:

struct strct;

struct strct *strct_create(void);
strct_foo(struct strct *self);
strct_bar(struct strct *self, int x);
[...]

and have the full declaration of struct strct in the file implementing these functions