How to link main with made libraries?

I'm trying to link libraries I made to main source code, but it throws this error. I've been searching for last two hours how to fix this but nothing worked for me.

Compiling:

cd "c:\Users\miros\OneDrive\Documents\C codes\labovi"
gcc 01main.c -o 01main
.\01main

d:/programs/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\miros\AppData\Local\Temp\ccGHtHKg.o:01main.c:(.text+0x46): undefined reference to 'faktorijel' d:/programs/mingw/bin/../lib/gcc/mingw32/9.2.0/../../../../mingw32/bin/ld.exe: C:\Users\miros\AppData\Local\Temp\ccGHtHKg.o:01main.c:(.text+0x72): undefined reference to 'suma' collect2.exe: error: ld returned 1 exit status

01ucitavanje.h

#ifndef UCITAVANJE_H
#define UCITAVANJE_H
#include <stdio.h>
#include <stdlib.h>

long long *niz;
int n;

void ucitavanje();
#endif

01ucitavanje.c

#include "01ucitavanje.h"

void ucitavanje(){
    scanf("%d", &n);
    niz = malloc(sizeof(long long) * n);
    for (int i = 0; i < n; i++)
        scanf("%d", niz + i);
}

01izracunavanje.h

#ifndef IZRACUNAVANJE_H
#define IZRACUNAVANJE_H
#include <stdio.h>

long long faktorijel(int n);
long long suma(long long *niz, int n);

#endif

01izracunavanje.c

#include "01izracunavanje.h"

long long faktorijel(int n){
    long long f;
    for (int i = 2; i <= n; i++)
        f *= i;

    return f;
}

long long suma (long long *niz, int n){
    long long s = niz[0];
    for (int i = 1; i < n; i++)
        s += niz[i];
    return s;    
}

main.c

#include "01izracunavanje.h"
#include "01ucitavanje.h"

int main(){
    ucitavanje();
    for (int i = 0; i < n; i++)
        niz[i] = faktorijel((int)niz[i]);
    printf("%d", suma(niz, n));
    free(niz);
    return 0;    
}

You're compiling only main.c into the executable, so the other functions aren't available.

You need to compile each source file into an object file, then link the object files together.

gcc -c main.c
gcc -c 01izracunavanje.c
gcc -c 01ucitavanje.c
gcc -o 01main main.o 01izracunavanje.o 01ucitavanje.o