pointer to pointer for struct

I have the following code

#include <stdlib.h>
#include <stdio.h>

typedef struct {
    int age;
} data;

int storage (data **str) {
    *str = malloc(4 * sizeof(**str));
    (*str)[0].age = 12;
    return 0;
}

int main() {
    data *variable = NULL;
    storage(&variable);
    return 0;
}

I took it from a website source. I think I have a misunderstanding about a basic pointer to pointer concept because here in this code, we have a pointer to a struct, variable, and we are passing this to storage function, which expects pointer to pointer of struct type. After memory was malloced, I don't understand this assignment

(*str)[0].age = 12

It was assigned as if, str was of (*)[] type. I dont understand how this assignment works, like str is now a pointer to an array of structs?