c Language Simple Problem Questions

I am a college student who is preparing to return to school after discharge. While preparing for my return to school, I was solving some C language problems, but there was a problem that got stuck while solving them, so I asked a question.

And then str1 and str2 are used to send text messages from the program user It receives input, but it receives input through an fgets function call. Subsequently Copy the string stored in str1 to str3 and save it to str2 Let's add the string after the string stored in str3. And Finally, let's print out the string stored in str3.

For example, strings stored in str1 and str2 are: 같다면, str1 "simple" str2 "string"

Str3 should contain the string "simplestring".

While solving these questions,

#include <stdio.h>

int main(void)
{
    char str1[20];
    char str2[20];
    char str3[40];

    fputs ("String input 1: ", stdout);
    fgets(str1, sizeof(str1), stdin);
    fputs ("String input 2: ", stdout);
    fgets(str2, sizeof(str2), stdin);
    str1[strlen(str1) - 1] = 0;
    str2[strlen(str2) - 1] = 0;
    for (int i = 0; i < strlen(str1); i++)
        str3[i] = str1[i];
    for (int i = 0; i < strlen(str2); i++)
        str3[strlen(str1)+i] = str2[i];
    printf("%s\n", str2);
    printf ("Result of combination: %s", str3);
    return 0;
}

I solved it like this.

조합의 결과 : simplestring儆儆儆儆儆儆儆儆儆儆儆儆儆儆儆儆儆儆string

It came out like this. Why is this weird string attached to the back? I wonder if the margin of str3 comes out like that, and why does the string stick at the end again If I don't use the strncpy function and do it like that, does it come out like this?