variable changing its value on its own in c

For context, I'm a beginner in C and I was making a file with all the important details of the language, and I noticed that a variable (type int) that I defined in the start seemed to change its value later on in the program when I accessed it using pointers.

here's how the code goes on (I removed all the comments that explained each line(bc Im a beginner))

#include <stdio.h>
#include <string.h>

int main() {

    printf("Hello World\n");

    int uninitialisedArrayOfInt[4];

    int initialisedArrayOfInt[] = {0, 1, 2, 3};


    int len = sizeof(uninitialisedArrayOfInt)/sizeof(int);


    for (int i = 0; i < len; i++){
        uninitialisedArrayOfInt[i] = i+1;
        initialisedArrayOfInt[i] = i+2;
    }



    int i = 0;

    while(i<len){
        printf("%i\n", uninitialisedArrayOfInt[i]);
        i++;
    }


    int arr[1][1];


    char str1[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '
#include <stdio.h>
#include <string.h>
int main() {
printf("Hello World\n");
int uninitialisedArrayOfInt[4];
int initialisedArrayOfInt[] = {0, 1, 2, 3};
int len = sizeof(uninitialisedArrayOfInt)/sizeof(int);
for (int i = 0; i < len; i++){
uninitialisedArrayOfInt[i] = i+1;
initialisedArrayOfInt[i] = i+2;
}
int i = 0;
while(i<len){
printf("%i\n", uninitialisedArrayOfInt[i]);
i++;
}
int arr[1][1];
char str1[] = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '!', '\0'}; 
printf("%s\n", str1);
char str2[] = "Hello World!";
printf("%s\n", str2);
int len2 = strlen(str1);
printf("%i\n", len2);
strcat(str1, str2); //the len values changes from this step to the next
printf("%s\n", str1); //len becomes 1919899424
char copycat[sizeof(str2)];
strcpy(copycat, str2);
printf("%s\n", copycat);
int* lenPtr = &len;
printf("%p\n", lenPtr);
printf("%i\n", *lenPtr);
*lenPtr = 200;
printf("%i\n", len);
}
'}; 
    printf("%s\n", str1);

    char str2[] = "Hello World!";
    printf("%s\n", str2);


    int len2 = strlen(str1);
    printf("%i\n", len2);



    strcat(str1, str2); //the len values changes from this step to the next
    printf("%s\n", str1); //len becomes 1919899424


    char copycat[sizeof(str2)];
    strcpy(copycat, str2);
    printf("%s\n", copycat);



    int* lenPtr = &len;
    printf("%p\n", lenPtr);



    printf("%i\n", *lenPtr);
    *lenPtr = 200;
    printf("%i\n", len);

}

The funny part here is that I play around with len only in the start and end and yet the value changes to 1919899424(it changes) on the line where I concatenate str1 and str2.

Just so you guys know I ran this code in vscode and the compiler and debugger I am using is gcc and gdb.

I had to include this entire code because without it the error does not occur

as many are confused the strcat() function does not change the value of len as explained here (https://www.codecademy.com/courses/learn-c-arrays-and-strings/lessons/strings-c/exercises/concatenating-strings#:~:text=What%20actually%20happens%20is%20the%20function%20takes%20the%20two%20strings%20and%20creates%20a%20char%20array%20of%20size%20strlen(src)%20%2B%20strlen(dst)%20%2B%201.)

There is no free space in str1 to concat other string. After concat you modify memory outside of str1 array and change the value of len variable (in this case).