As a sanity check, does this work?

#include <curl/curl.h>
#include <stdio.h>
#include <stdlib.h>

size_t save_to_disk(char* ptr, size_t size, size_t nmemb, void* user_data)
{
    /* according to curl's docs size is always 1 */
    
    FILE* fp = (FILE*)user_data;
    fprintf(stderr, "got %lu bytes\n", nmemb);
    return fwrite(ptr, size, nmemb, fp);
}

int main(int argc, char* argv[])
{
    char errbuf[CURL_ERROR_SIZE];
    FILE* fp = NULL;
    CURLcode res;
    
    CURL* curl = curl_easy_init();
    
    if(!curl)
    {
        fprintf(stderr, "Failed to initialize curl\n");
        return EXIT_FAILURE;
    }
    
    fp = fopen("output.data", "wb");
    if(!fp)
    {
        fprintf(stderr, "Failed to open file for writing!");
        return EXIT_FAILURE;
    }
    
    curl_easy_setopt(curl, CURLOPT_URL, "https://www.wikipedia.org");
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, save_to_disk);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
    curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errbuf);
    
    errbuf[0] = 0;  /* set error buffer to empty string */
    res = curl_easy_perform(curl);
    
    if(fp)
    {
        fclose(fp);
        fp = NULL;
    }
    
    if(res != CURLE_OK)
    {
        fprintf(stderr, "error code   : %d\n", res);
        fprintf(stderr, "error buffer : %s\n", errbuf);
        fprintf(stderr, "easy_strerror: %s\n", curl_easy_strerror(res));
        
        return EXIT_FAILURE;
    }
    else
    {
        fprintf(stderr, "\nDone\n");
        return EXIT_SUCCESS;
    }
}

That should write a file called output.data with the HTML from https://www.wikipedia.org and print out the number of bytes each time the write callback receives data for processing.

On my machine, it prints the following when it works successfully (byte counts may vary for you):

got 13716 bytes
got 16320 bytes
got 2732 bytes
got 16320 bytes
got 16320 bytes
got 128 bytes
got 16320 bytes
got 16320 bytes
got 1822 bytes

Done

If I change the URL to nonsense instead to make it fail, it prints text like this on my system:

error code   : 6
error buffer : Could not resolve host: nonsense
easy_strerror: Couldn't resolve host name

Edit: corrected missing line in source (i.e. added line with CURLOPT_ERRORBUFFER which is needed to get extra info in the error buffer on failure, of course)

Edit 2: tweaks to wording to try to be more clear