Source

File: wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php

public function get_item( $request ) {
		$options  = $this->get_registered_options();
		$response = array();

		foreach ( $options as $name => $args ) {
			/**
			 * Filters the value of a setting recognized by the REST API.
			 *
			 * Allow hijacking the setting value and overriding the built-in behavior by returning a
			 * non-null value.  The returned value will be presented as the setting value instead.
			 *
			 * @since 4.7.0
			 *
			 * @param mixed  $result Value to use for the requested setting. Can be a scalar
			 *                       matching the registered schema for the setting, or null to
			 *                       follow the default get_option() behavior.
			 * @param string $name   Setting name (as shown in REST API responses).
			 * @param array  $args   Arguments passed to register_setting() for this setting.
			 */
			$response[ $name ] = apply_filters( 'rest_pre_get_setting', null, $name, $args );

			if ( is_null( $response[ $name ] ) ) {
				// Default to a null value as "null" in the response means "not set".
				$response[ $name ] = get_option( $args['option_name'], $args['schema']['default'] );
			}

			/*
			 * Because get_option() is lossy, we have to
			 * cast values to the type they are registered with.
			 */
			$response[ $name ] = $this->prepare_value( $response[ $name ], $args['schema'] );
		}

		return $response;
	}