Only the record title can be used to search for records in the record finder. By default, the linked table's primary field is the record title. You can show or search using fields other than the primary field in the linked table, by selecting a custom title field for linked records. By doing so, the record finder will search that field instead. You can still choose to display the original primary field as a preview field, if needed for reference.
To scan barcodes or QR codes in a specific field, have a look at this help article.