For example, when we create a mobile number customer attribute in Magento 2,  we need to create a module to add new customer attribute in Magento2.

Create a registration.php file to the following path
app/code/Magemonkeys/CustomerAttribute/registration.php

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkeys_CustomerAttribute',
    __DIR__
);

Create module.xml file to the following path  app/code/Magemonkeys/CustomerAttribute/etc/module.xml which define our module setup version to setup_module table.

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
  <module name="Magemonkeys_CustomerAttribute" setup_version="1.0.0">
    <sequence>
      <module name="Magento_Customer"/>
    </sequence>
  </module>
</config>

Then, we need to create an extension_attributes.xml file to the following path app/code/Magemonkeys/CustomerAttribute/etc/extension_attributes.xml

which defines our mobile number customer attribute.

Pass Classname in extension_attributes.xml as MagentoCustomerApiDataCustomerInterface because its a base Customer interface to create customer attributes in Magento2.

Set attribute code as your customer attribute code and define type as a string.

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="MagentoCustomerApiDataCustomerInterface">
        <attribute code="mobile" type="string"/>
    </extension_attributes>
</config>

For Create Customer attribute we need to define InstallData.php file to the following path app/code/Magemonkeys/CustomerAttribute/Setup/InstallData.php which adds an entry to our database for custom mobile number attribute.

<?php

namespace MagemonkeysCustomerAttributeSetup;

use MagentoFrameworkSetupInstallDataInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupModuleDataSetupInterface;
use MagentoCustomerModelCustomer;
use MagentoCustomerSetupCustomerSetupFactory;

class InstallData implements InstallDataInterface
{

    private $customerSetupFactory;

    /**
     * Constructor
     *
     * @param MagentoCustomerSetupCustomerSetupFactory $customerSetupFactory
     */
    public function __construct(
        CustomerSetupFactory $customerSetupFactory
    ) {
        $this->customerSetupFactory = $customerSetupFactory;
    }

    /**
     * {@inheritdoc}
     */
    public function install(
        ModuleDataSetupInterface $setup,
        ModuleContextInterface $context
    ) {
        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

        $customerSetup->addAttribute(MagentoCustomerModelCustomer::ENTITY, 'mobile', [
            'type' => 'varchar', // type of attribute
            'label' => 'Mobile Number',
            'input' => 'text', // input type
            'source' => '',
            'required' => false, // if you want to required need to set true
            'visible' => true,
            'position' => 500, // position of attribute
            'system' => false,
            'backend' => ''
        ]);
        
        /* Specify which place you want to display customer attribute */
        $attribute = $customerSetup->getEavConfig()->getAttribute('customer', 'mobile')
        ->addData(['used_in_forms' => [
                'adminhtml_customer',
                'adminhtml_checkout',
                'customer_account_create',
                'customer_account_edit'
            ]
        ]);
        $attribute->save();
    }
}

Now Run the upgrade command to install our module using SSh from the root of your Magento instance.

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f
php bin/magento indexer:reindex
php bin/magento cache:flush

Now you can check your new mobile number attribute at the admin panel.

Go To, Customers -> All Customers -> Edit any Customer.