When creating new orders, the system automatically determines which inventory location to source the item from. Shopify or Centra, depending on your setup, are responsible for making these inventory decisions. The system typically defaults to your primary location, as specified in your settings.

We check the total quantity of a variant across all your inventories in Shopify or Centra. This means that even if the e-shop shows no stock for a variant, but it’s available in your physical store, we still allow exchanges—as long as it’s in stock in your back office.
​Note: If inventory levels are low, consider approving exchange requests promptly.

For exchanges, it's necessary to manually restock the items from the original order upon their return in your OMS.

You can check how the priority is set up here https://domain-name.myshopify.com/admin/settings/locations (replace domain-name with your store name) as we don't have access for this

Also here is a detailed document about how location fulfilment actually works in Shopify, as there are many possible alternatives not just primary/default location https://help.shopify.com/en/manual/locations/setting-up-your-location-fulfillments#order-fulfillment-priority.

The set-up the default location and prevent other locations to fulfil online orders -"If you want all items for online orders to fulfil from a single location, then you need to make that your default location and prevent other locations from fulfilling online orders".