Magento – Magento 2 checkout page how to get country_id from selected shipping addresses

In order to show the region fields should be shown for non required state :

(can be configured from Stores / Configuration / General / State Options / Allow to Choose State if it is Optional for Country)

vendor/magento/module-directory/Helper/Data.php:268

/*
 * Path to config value, which detects whether or not display the state for the country, if it is not required
 */
const XML_PATH_DISPLAY_ALL_STATES = 'general/region/display_all';

/**
 * Return, whether non-required state should be shown
 *
 * @return bool
 */
public function isShowNonRequiredState()
{
    return (bool)$this->scopeConfig->getValue(
        self::XML_PATH_DISPLAY_ALL_STATES,
        ScopeInterface::SCOPE_STORE
    );
}

In order to set the region required for a country :

(can be configured from Stores / Configuration / General / State Options / State is Required for)

vendor/magento/module-directory/Helper/Data.php:281

/*
 * Path to config value, which lists countries, for which state is required.
 */
const XML_PATH_STATES_REQUIRED = 'general/region/state_required';

/**
 * Returns flag, which indicates whether region is required for specified country
 *
 * @param string $countryId
 * @return bool
 */
public function isRegionRequired($countryId)
{
    $countyList = $this->getCountriesWithStatesRequired();
    if (!is_array($countyList)) {
        return false;
    }
    return in_array($countryId, $countyList);
}

Then to add a new country and region :

1. Add a now country inside this table : directory_country
2. Add related region inside this table : directory_country_region related with the country_id column.
3. Add translated version of your region in this table : directory_country_region_name

Add these information from a setup script. You can based on the Magento Directory Setup Data : vendor/magento/module-directory/Setup/InstallData.php:17

To add the dropdowns in your custom module for checkout. You can use /vendor/magento/module-checkout/Block/Checkout/DirectoryDataProcessor.phpas reference.

You need to create a plugin for the \Magento\Checkout\Block\Checkout\DirectoryDataProcessor::process method. In your plugin, implement the after method adding the option data to the corresponding layout nodes at run-time.

Create di.xml to declare the plugin

<?xml version='1.0'?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd" >
    <type name="Magento\Checkout\Block\Checkout\DirectoryDataProcessor">
        <plugin name='customDropdownPlugin' type='VendorName\ModuleName\Plugin\Block\Checkout\DirectoryDataProcessorPlugin' sortOrder='999'/>
    </type>
</config>

Create VendorName\ModuleName\Plugin\Block\Checkout\DirectoryDataProcessorPlugin.php

<?php
namespace VendorName\ModuleName\Plugin\Block\Checkout;


class DirectoryDataProcessorPlugin
{
    /**
     * @param \Magento\Checkout\Block\Checkout\DirectoryDataProcessor $subject
     * @param array $jsLayout
     * @return array
     */
    public function afterProcess(
        \Magento\Checkout\Block\Checkout\DirectoryDataProcessor $subject,
        array  $jsLayout
    ) {
        if (isset($jsLayout['components']['checkoutProvider']['dictionaries'])) {
            $jsLayout['components']['checkoutProvider']['dictionaries'] = [
                'custom_country_id' => $subject->getCountryOptions(),
                'custom_region_id' => $subject->getRegionOptions(),
            ];
        }

        return $jsLayout;
    }
}

Here you need to replace custom_cuntry_id and custom_region_id with your fileds respectively.

Also check your attribute you have created must be of type select in eav_attribute table for customer address entity.