Practice to use memcpy or strncpy to copies the string into memory

I am writing a piece of simple practice to compare the usage of between memcpy or strncpy. I see that I can do this string copy using either of these two functions:

char * strncpy ( char * destination, const char * source, size_t num );

and

void * memcpy ( void * destination, const void * source, size_t num );

Simple practice

    char src[] = "Hello World";
    char dst[20];

    strncpy(dst, src, 5);
    printf("%zu-", strlen(dst));

    memcpy(dst, src, sizeof(dst));
    printf("%zu-", strlen(dst));

    strncpy(dst, src, 5);
    printf("%zu", strlen(dst));

The actually output

5-11-11

How can I understand what the behave differently with respect?

In general, how can I find out relative bytes of memory block different copied by using memcpy or strncpy?

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