Static Classes without Implementation

What do static classes without implementation mean? Like this:

    public static class Host
    {
        public static HostApplicationBuilder CreateApplicationBuilder();
        ...
    }

    public static class Host
    {
        public static HostApplicationBuilder CreateApplicationBuilder();
        ...
    }

Does the runtime contain implementations and resolve them dynamically at runtime? Where/How can I read the code for these functions then?

Static Classes without Implementation - C#