If you want to have all the code in a C file

what that single file would look like:

// includes: (proyect includes with "" and system includes with <>)
#include "a.h"
...
#include "headers/n.h"
...
// system includes:
#include <stdio.h>
...

// structs:
typedef struct {
    typeData variable_name1;
    ...
    typeData variable_nameN;
    struct name_type* var_name;
} A;
...
typedef struct {
    typeData variable_name1;
    ...
    typeData variable_nameN;
    struct name_type* var_name;
} N;
// alias:
typedef oldType newType;
...
typedef char* String;
// prototypes:
void fuction1(A* param);// prototype
...
// implementations:
N* fuctionN(int param1, char param2);// prototype
// functions on structs:
void function1(A* param) {
...
}

N* fuctionN(int param1, char param2){
...
}

remember:

typedef oldType newType;

It is to define aliases, to write something easier and invoke the same thing, it is a syntactical replacement

with # preprocessor directives

The following example defines a symbol that is unique:

#ifndef UNIQUE_NAME_FILE_H
#define UNIQUE_NAME_FILE_H

typedef char* String; // other alias for example

// Structure representing a Aname
typedef struct {
    String text;
    typeData field1;
    int field2;
} Aname;

#endif // UNIQUE_NAME_FILE_H

You define structs in a .h, function implementations in a .c, and prototypes functions along with the #includes in a .h

complete example

#include <stdio.h>

// Function prototype
int add(int a, int b);

int main() {
    int result = add(5, 3);
    printf("the addition is: %d\n", result);
    return 0;
}

// Implementation
int add(int a, int b) {
    return a + b;
}

The single file extension is .c (.h is for sharing responsibility and neatness)

---

example:

1 single file:

https://gitlab.com/com.leibnix/allinone

organized (several files):

https://gitlab.com/com.leibnix/organized_c