Explanation
Two possible reasons that could cause the test org to not appear in the list of target organizations are:
The components in the Change set could not be found within the test org. This means that the test org does not have the same metadata components or features as the development org, and therefore cannot receive the change set.
The development org has not been approved to upload Change Sets from within the test org. This means that there is no deployment connection between the two orgs, and the test org has not authorized the development org to send change sets to it.
Verified References: https://help.salesforce.com/s/articleView?id=sf.changesets_about.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.changesets_create_outbound.htm&type=5

Explanation
The objects that will be created in Order Management when a customer orders a product through B2C Commerce but changes the quantity ordered after the Order Summary record is created in Order Management, and then returns one of the products are:
A Return Order object. A Return Order is a record that represents a return request for an order or part of an order. A Return Order has a lookup relationship to both Order Summary and Change Order objects, and it contains information such as the return reason, status, date, etc.
A Change Order object. A Change Order is a record that represents a change request for an order or part of an order. A Change Order has a lookup relationship to the Order Summary object, and it contains information such as the change type, status, date, etc.
Verified References:
https://help.salesforce.com/s/articleView?id=sf.order_management_return_order.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.order_management_change_order.htm&type=5

A service agent should go first to the Order Summary Details page to initiate changes related to an order in Order Management. The Order Summary Details page provides a global view of the entire order lifecycle, including order capture, fulfillment, shipping, payment, invoicing, and service. From this page, a service agent can access various actions and flows to process changes such as cancellations, returns, exchanges, reshipments, refunds, and discounts. The service agent can also view related records such as Order, Fulfillment Orders, Shipments, Invoices, Credit Memos, and Change Orders. Reference: Order Management Console, Order Management Lifecycle

The Ensure Funds process works on only the items passed into the Ensure Funds method. This method is a flow core action that verifies that there are enough funds available for each Order Item passed into it, and updates their payment status accordingly. The administrator can pass any Order Items into this method, regardless of their status or delivery group. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_om_actions_ensure_funds_async.htm&type=5