How to Interpret the Inheritance in Variadic Template?

I came across this post: Convert a number to a string literal with constexpr

And the answer is quite interesting:

namespace detail
{
    template<unsigned... digits>
    struct to_chars { static const char value[]; };

    template<unsigned... digits>
    const char to_chars<digits...>::value[] = {('0' + digits)..., 0};

    template<unsigned rem, unsigned... digits>
    struct explode : explode<rem / 10, rem % 10, digits...> {};

    template<unsigned... digits>
    struct explode<0, digits...> : to_chars<digits...> {};
}

template<unsigned num>
struct num_to_string : detail::explode<num> {};

My questions are:

1. "struct explode : explode" declares explode inherits from explode; how about "struct explode<0, digits...> : to_chars"?

2. What's the function for the '0' as the first template parameter?

Thanks!