cells in battery packs are organised in an array, in your case, 13S4P.
Each string has 13 cells in series, 4 strings are paralleled together to form the pack.
So for each string, you have 13 x 3.7V =48.1V, 3000mAH.
For the pack, you have 48.1V, 4x3000mAH = 12AH.