"Item is a Record" column is not showing under site columns. We are able to see this column into view also able to add this column into "All Documents" view but unable to use value of this column.
It is by designer that "item is a Record" column is not showing under site columns.
For more inforamtion about “Item is a Record” column, refer to the following articles.
Modern vs Classic IN PLACE Records Management in SharePoint for your reference:
Overview of retention labels: