JSFiddle - React, Tailwind, and code Playground

JavaScript

const getAllTerritoryCodesFromRegions = array => {
    const output = [];

    array.forEach(item => {
        for (const key in item) {
            if (key === 'code') {
                output.push(item[key]);
            } else if (item.hasOwnProperty(key) && Array.isArray(item[key])) {
                const childOutput = getAllTerritoryCodesFromRegions(item[key]);
                output.push(...childOutput);
            }
        }
    });

    return output;
}

const regions = [{
  name: 'Europe',
  subRegions: [{
    name: 'BeNeLux',
    territories: [{
      code: 'NL',
      name: 'Netherlands'
    }, {
      code: 'DE',
      name: 'Germany'
    }, {
      code: 'LU',
      name: 'Luxembourgh'
    }]
  }],
  territories: [{
    name: 'United Kingdom',
    code: 'UK'
  }, {
    name: 'AL',
    code: 'Albania'
  }, {
    name: 'ZW',
    code: 'Switzerland'
  }]
}]

const result = getAllTerritoryCodesFromRegions(regions)

console.log(result);