I'm having a hard time understanding the editorial: https://atcoder.jp/contests/abc221/editorial/2732

for the problem: https://atcoder.jp/contests/abc221/tasks/abc221_e

I want to know how fen-wick tree is actually working in this problem. specially this block

for(int i=0; i<N; i++){
        ans += bit.sum(A[i])*modpow(2,i);
        ans %= mod;
        bit.add(A[i],modpow(div,i+1));
    }

I'm actually very new to CP and have no peers to seek help. So, I directly ask my problem through the blogs. please help me if you have an explanation to get me through it. Thanking you.