When you integrate Drupal and Magento, you need to decide where product data lives and where marketing data lives. A common approach to generating this breakdown is to mirror the organization’s team structure. If a content team handles promotional copy, images, and visual / editorial design, then these fields should be sourced from Drupal. If a merchandising team determines inventory and pricing, manages stock levels, and designs promotions, these fields should live in Drupal. Using a breakdown like this means that each team only needs a set of keys to one of the platforms. Also, any disruption in one of the systems is unlikely to affect the other.
Technically, we will choose Magento to source of truth for product SKU. So, any CRUD for products will be synced from Magento to Drupal with Asynchronous communication based on Event Queue + API call.
Any redirect page from Magento to Drupal that will be rewrite URL from rewrite table or from code by observer.
This is for all about side by side solution for Magento + Drupal. We also have some other way like Headless solution for this that allow to combination Magento + Drupal through APIs. But it will be another topic.