Is Array of Objects reactive in VueJs 2.7?

Why is it reactive, I haven't find any info about it in documentation

updateFieldSelectedArray(state, { index, key, value }) {
      state.selectedArray[index][key] = value;
    },

selectedArray is Array of Objects and this code is reactive but THIS ->

updateFieldSelectedArray(state, { index }) {
      state.selectedArray[index] = someRandomValue;
    },

wouldn't be reactive