There are four main types of Customizer objects: Panels, Sections, Settings, and Controls. To add, remove, or modify any Customizer object, and to access the Customizer Manager, we have to use the customize_register hook.

//example of customize_register hook

function themeslug_customize_register( $wp_customize ) {
  // Do stuff with $wp_customize, the WP_Customize_Manager object.
}
add_action( 'customize_register', 'themeslug_customize_register' );

//adding section, setting & controls

function dws_customize_register($wp_customize){

$wp_customize->add_section('dws_color_scheme', array(
'title'    => __('Color Scheme', 'theme_name'),
'description' => '',
'priority' => 120,
));

// Text Input

$wp_customize->add_setting('dws_theme_options[text_test]', array(
'default'        => 'value abc',
'capability'     => 'edit_theme_options',
'type'           => 'option',

));

$wp_customize->add_control('dws_text_test', array(
'label'      => __('Text Test', 'theme_name'),
'section'    => 'dws_color_scheme',
'settings'   => 'dws_theme_options[text_test]',
));

//  Radio Input 

$wp_customize->add_setting('dws_theme_options[color_scheme]', array(
'default'        => 'value2',
'capability'     => 'edit_theme_options',
'type'           => 'option',
));

$wp_customize->add_control('dws_color_scheme', array(
'label'      => __('Color Scheme', 'theme_name'),
'section'    => 'dws_color_scheme',
'settings'   => 'dws_theme_options[color_scheme]',
'type'       => 'radio',
'choices'    => array(
'value1' => 'Choice 1',
'value2' => 'Choice 2',
'value3' => 'Choice 3',
),
));

// Checkbox
$wp_customize->add_setting('dws_theme_options[checkbox_test]', array(
'capability' => 'edit_theme_options',
'type'       => 'option',
));

$wp_customize->add_control('display_header_text', array(
'settings' => 'dws_theme_options[checkbox_test]',
'label'    => __('Display Header Text'),
'section'  => 'dws_color_scheme',
'type'     => 'checkbox',
));
}

add_action('customize_register', 'dws_customize_register');