Fields

The WordPress Customizer API allows you to add a new control like this:

function my_custom_text_settings( $wp_customize ) {

	// Register the settings
	$wp_customize->add_setting( 'my_setting', array(
		'default'        => 'some-default-value',
		'type'           => 'theme_mod',
		'capability'     => 'edit_theme_options',
	) );

	$wp_customize->add_setting( 'my_setting_2', array(
		'default'        => 'some-default-value',
		'type'           => 'theme_mod',
		'capability'     => 'edit_theme_options',
	) );

	// Add the controls
	$wp_customize->add_control( 'my_setting', array(
		'label'       => __( 'My custom control', 'translation_domain' ),
		'section'     => 'my_section',
		'settings'    => 'my_setting',
		'type'        => 'text',
		'priority'    => 10
    ) );

	$wp_customize->add_control( 'my_setting_2', array(
		'label'       => __( 'My custom control 2', 'translation_domain' ),
		'section'     => 'my_section',
		'settings'    => 'my_setting_2',
		'type'        => 'text',
		'priority'    => 20
    ) );
}
add_action( 'customize_register', 'my_custom_text_settings' );

You can do the exact same thing using Kirki like this:

Kirki::add_config( 'my_config', array(
	'options_type' => 'theme_mod',
	'capability'   => 'edit_theme_options',
) );

Kirki::add_field( 'my_config', array(
	'settings' => 'my_setting',
	'label'    => __( 'My custom control', 'translation_domain' ),
	'section'  => 'my_section',
	'type'     => 'text',
	'priority' => 10,
	'default'  => 'some-default-value',
) );

Kirki::add_field( 'my_config', array(
	'settings' => 'my_setting_2',
	'label'    => __( 'My custom control 2', 'translation_domain' ),
	'section'  => 'my_section',
	'type'     => 'text',
	'priority' => 20,
	'default'  => 'some-default-value',
) );

WordPress core allows you to add a lot of fields in your customizer. This way you can add options to your theme without using an options framework like you needed a few years ago.

We recommend you read this blog post from Otto to learn the basics of how to use the WordPress customizer. and the Handbook on WordPress.org.