How does the function printf works in C?

I met a problem when I test the function printf:

First I write code like this:

int main(void)
{
    char a = 'a';
    printf("a = %f\n", a);
    return 0;
}

The output is

And then I write code:

int main(void)
{
    float b = 'a';
    printf("b = %f\n", b);
    return 0;
}

The output is

And then I write code:

int main(void)
{
    char a = 'a';
    float b = 'a';
    printf("b = %f\n", b);
    printf("a = %f\n", a);
    return 0;
}

The output is

So I am confused why in the first program a = 0.000000 and in the third program a = 97.000000?

How does the function printf() work?

How does the symbol %f, %d work?