Note that we will have the following
As EP said ....after the first pass each subsequent pass places 6 bags
So
[2021 - 7] / 6 = 335 additional passes after the first pass plus 4 additional bags
Let the first pass = pass 0
And notice that after 5 more passes we end up in box 1
pass 0 1 2 3 4 5
1 2 3 4 5 6 7 / 6 5 4 3 2 1 / 2 3 4 5 6 7 / 6 5 4 3 2 1 / 2 3 4 5 6 7 / 6 5 4 3 2 1 /
We have 335 additional passes after the first pass....so there will be 335/5 = 67 "red" cycles with 30 boxes in each cycle
And at the start of the 68th cycle we will place 4 more bags in boxes 2 3 4 5
So...we end up in box 5.....just as EP found
Proof
7 + 30*67 + 4 = 2021 boxes filled