Source

File: wp-admin/includes/class-wp-automatic-updater.php

public function is_disabled() {
		// Background updates are disabled if you don't want file changes.
		if ( ! wp_is_file_mod_allowed( 'automatic_updater' ) ) {
			return true;
		}

		if ( wp_installing() ) {
			return true;
		}

		// More fine grained control can be done through the WP_AUTO_UPDATE_CORE constant and filters.
		$disabled = defined( 'AUTOMATIC_UPDATER_DISABLED' ) && AUTOMATIC_UPDATER_DISABLED;

		/**
		 * Filters whether to entirely disable background updates.
		 *
		 * There are more fine-grained filters and controls for selective disabling.
		 * This filter parallels the AUTOMATIC_UPDATER_DISABLED constant in name.
		 *
		 * This also disables update notification emails. That may change in the future.
		 *
		 * @since 3.7.0
		 *
		 * @param bool $disabled Whether the updater should be disabled.
		 */
		return apply_filters( 'automatic_updater_disabled', $disabled );
	}