Plugins are included as it is, without any modification in plugin itself.
Few overrides via theme, which can be undo (not necessary though) easily.
On ThemeForest, when a premium plugin is included in a theme for free you will receive plugin update only if theme is updated for that plugin. Plugins for this theme are updated as and when required. Not essentially as soon as plugin is updated by its original developer, but considering the updates made to the plugin and making it compatible with the theme.
If you need latest updates and support for the plugin directly from plugin author it will need to purchase it separately, this is purely optional.
Theme do not use each and every functionality provided by plugin. In case, any bug generated by plugin is reported, please notify via support form, we will resolve it by contacting plugin developer.