How to add a custom column in customer table in Magento2?

You can create custom customer Attribute Adding Customer Attribute

1) Create the Module file

<?xml version="1.0" encoding="UTF-8"?>
<config  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
   <module name="Kalpesh_Mobile"  setup_version="1.0.0">
       <sequence>           
            <!--<module name="Kalpesh_Mobile"/>-->
            <module name="Magento_Customer"/>
        </sequence>   
   </module>
</config>



class InstallData implements InstallDataInterface
    {
        protected $customerSetupFactory;

        /**
         * @var AttributeSetFactory
         */
        private $attributeSetFactory;

        /**
         * @param CustomerSetupFactory $customerSetupFactory
         * @param AttributeSetFactory $attributeSetFactory
         */
        public function __construct(
            CustomerSetupFactory $customerSetupFactory,
            AttributeSetFactory $attributeSetFactory
        ) {
            $this->customerSetupFactory = $customerSetupFactory;
            $this->attributeSetFactory = $attributeSetFactory;
        }

        public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
        {
                    $setup->startSetup();


            /** @var CustomerSetup $customerSetup */
            $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

            $customerEntity = $customerSetup->getEavConfig()-
      >getEntityType('customer');
            $attributeSetId = $customerEntity->getDefaultAttributeSetId();

            /** @var $attributeSet AttributeSet */
            $attributeSet = $this->attributeSetFactory->create();
            $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);

            $customerSetup->addAttribute(Customer::ENTITY, 'mobile', [
                'type' => 'varchar',
                'label' => 'Mobile',
                'input' => 'text',
                'required' => false,
                'visible' => true,
                'user_defined' => true,
                'sort_order' => 1000,
                'position' => 1000,
                'system' => 0,
            ]);

            $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'mobile')
            ->addData([
                'attribute_set_id' => $attributeSetId,
                'attribute_group_id' => $attributeGroupId,
                'used_in_forms' => ['customer_address_edit'],
            ]);

            $attribute->save();

                    $setup->endSetup();



        }
    }

You need to create your own module

in Magento simple module example You can check the https://github.com/magento/magento2-samples/blob/master/sample-module-form-uicomponent/view/adminhtml/ui_component/sampleform_form.xml they have provided functional for adding a new field

<field name="color">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <!--component constructor-->
            <item name="component" xsi:type="string">Magento_SampleForm/js/form/element/color-select</item>
            <!--main template for form field that renders elementTmpl as a child template-->
            <item name="template" xsi:type="string">ui/form/field</item>
            <!--customized form element template that will show colors-->
            <item name="elementTmpl" xsi:type="string">Magento_SampleForm/form/element/color-select</item>
            <item name="label" xsi:type="string">Autumn colors</item>
            <item name="visible" xsi:type="boolean">true</item>
            <item name="dataType" xsi:type="string">text</item>
            <item name="formElement" xsi:type="string">input</item>
            <item name="source" xsi:type="string">sampleform</item>
        </item>
    </argument>
</field>

in m2 there is no need to edit mysql rows directly or change core code, everything you could and suppose to rewrite. Read docs about general principles of working with magento 2

and as mentioned in comments if you need telephone field it's already implement