Classic Commerce is a fork of WooCommerce 3.5.3 and, as a first step, it is recommended that you refer to the WooCommerce docs. Most of this information will still be applicable to Classic Commerce:
All extensions compatible with WooCommerce 3.5.3 should still be usable provided they do not rely on Jetpack or WooCommerce Services. However, many plugins and extensions check for the existence of woocommerce/woocommerce.php in the plugins folder prior to their activation. To circumvent this issue we have created a special compatibility plugin called “CC Compatibility for Woo Addons”.
Snippets – these add minor functionality without the need for extra plugins.