Three steps that are required when deploying changes via change sets are:
Create the change set in the sandbox org. This is the first step where the administrator selects the metadata components that they wantto deploy and adds them to a change set in the source org, which is usually a sandbox org.
Upload the change set to the target org. This is the second step where the administrator sends the change set from the source org to the destination org, which is usually a production org or another sandbox org. The administrator needs to have a deployment connection with the target org and permission to upload change sets.
Deploy the change set in the target org. This is the final step where the administrator validates and deploys the change set in the destination org. The administrator needs to have permission to deploy change sets and resolve any deployment errors or warnings.
Verified References: https://help.salesforce.com/s/articleView?id=sf.changesets_create_outbound.
htm&type=5 https://help.salesforce.com/s/articleView?id=sf.changesets_deploy.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 orderlifecycle, 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. References: Order Management Console, Order Management Lifecycle

The reason why Fulfillment Order does not appear as an option when creating a rollup field on the Order Summary object is that the Order Summary field on the Fulfillment Order object is a Lookup. A Lookup field is a type of custom field that creates a relationship between two objects, but does not support rollup summary fields. To create a rollup summary field, the relationship between the objects must be master-detail, which means that the child record cannot exist without its parent record. Verified References: https://help.salesforce.
com/s/articleView?id=sf.custom_field_types.htm&type=5https://help.salesforce.com/s/articleView?id=sf.
fields_about_roll_up_summary_fields.htm&type=5