Copy line items to another deal: how cloning works

Short answer

HubSpot's KB: when you create a quote from a deal, HubSpot clones the deal's line items into the quote as new, independent line items; when the quote is finalized, they sync back. Each cloned line item is associated only with its new parent. There is no ongoing sync between records, so edit each record's line items separately. Importing line items does not update the deal amount.