Logic about recursive return

Question

Hello guys I am studying binary tree search and recursiveness is very used in this type of data.

I have a question, for example, I did the following function to search for an element in the tree:

int busca_Abb(Abb* r, int busca)
{
    if(r == NULL) 
       return 0;
    if(r->chave == busca) 
       return 1;
    if(r->chave > busca)
        busca_Abb(r->esq, busca); 
      busca_Abb(r->dir,busca);      
}

But how is it possible if the function when called within itself has no integer variable receiving the returned value?

Complete code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define COUNT 15

typedef struct _abb Abb;
struct _abb{
    int chave;
    Abb* esq;
    Abb* dir;
};

Abb* criar_Abb(void)
{
    return NULL;
}

Abb* criarNo(int chave)
{
    Abb* novo = (Abb* ) malloc(sizeof(Abb));
    novo->chave=chave;
    novo->dir=novo->esq=NULL;
    return novo;
}

Abb* insere_Abb(Abb* r, int chave)
{
    if(r == NULL)
      return criarNo(chave);
    else if(r->chave > chave)
      r->esq = insere_Abb(r->esq, chave);
    else if(r->chave < chave)
      r->dir = insere_Abb(r->dir, chave);

    return r;
}

void print2DUtil(Abb *root, int space)
{
    int i;
    if (root == NULL)
        return;
    space += COUNT;
    print2DUtil(root->dir, space);
    printf("\n");
    for (i = COUNT; i < space; i++)
        printf(" ");
    printf("%d\n", root->chave);
    print2DUtil(root->esq, space);
}

int busca_Abb(Abb* r, int busca)
{
    if(r == NULL) 
       return 0;
    if(r->chave == busca) 
       return 1;
    if(r->chave > busca)
        busca_Abb(r->esq, busca); 
      busca_Abb(r->dir,busca);      
}

int main(void)
{
    Abb* r;
    r = criar_Abb();
    int bus;

    r = insere_Abb(r, 23);
    r = insere_Abb(r, 12);
    r = insere_Abb(r, 15);
    r = insere_Abb(r, 25);
    r = insere_Abb(r, 8);

     printf("Arvore criada...\n");
     print2DUtil(r, 0);

     // procurar 25
    bus = busca_Abb(r, 25);
    if(bus)
       printf("Achou o %d!!\n",25);
    else
       printf("Nao achou o %d!!\n",25);

    //procurar 403
    bus = busca_Abb(r, 403);
    if(bus)
       printf("Achou o %d!!\n", 403);
    else
       printf("Nao achou o %d!!\n", 403);


    return 0;
}