The type of identifier when using structured binding with 'tuple-like' binding protocol

Question

I'm learning structured binding, I'm confused with the type of identifier about "tuple-like" binding protocol, for example:

    int a = 3;
    double b = 2.1;

    tuple<int, double&> tpl(a, b);
    auto [a1, b1] = tpl;
    auto& [a2, b2] = tpl;
    auto&& [a3, b3] = tpl;
    const auto& [a4, b4] = tpl;
    using Ta1 = decltype(a1);    // this is 'int', 
    using Tb1 = decltype(b1);    //         'double&'
    using Ta2 = decltype(a2);    //         'int'
    using Tb2 = decltype(b2);    //         'double&'
    using Ta3 = decltype(a3);    //         'int'
    using Tb3 = decltype(b3);    //         'double&'
    using Ta4 = decltype(a4);    //         'const int'
    using Tb4 = decltype(b4);    //         'double &'

first, why Tb4 is double&, but not const double&?

second, is there any difference between auto& and auto&&? when should I use & or &&?

Thanks for your answer!