Array.map

'use strict'
var data1 = [
  ['A1', 'B1', 'C1', 'Key-A001', 'Key-A002', 'Key-A005', '', ''],
  ['A2', 'B2', 'C2', 'Key-A001', 'Key-A004', '', '', ''],
  ['xx', 'yy', 'zz', 'Key-B002', 'Key-A004', 'Key-X999', 'Key-A002', '']
]
var data2 = [
  ['val_a1', 'I am valueA1', 'A001'],
  ['val_a2', 'I am valueA2', 'A002'],
  ['val_a3', 'I am valueA3', 'A003'],
  ['val_a4', 'I am valueA4', 'A004'],
  ['val_a5', 'I am valueA5', 'A005'],
  ['val_B_one', 'I am valueB1', 'B001'],
  ['val_B_two', 'I am valueB2', 'B002']
]
var data3 = data1.map(e => {
  return e.map((e1, i) => {
    var key = e1.match(/\w\d+$/)
    var found
    if (key) {
      found = data2.find(e2 => e2[2] === key[0])
    }
    if (i < 3 || !key || !found) {
      return e1
    }
    return `${found[0]}<br>(${found[2]})`
  })
})
console.log(data3)