_fivetran_id columns in the destination schema?
The two system columns are created by Fivetran to work with the tables that do not have any primary keys for them.
fivetran_index(INTEGER) enables the user to make sense of the order of updates for primary keyless tables.
_fivetran_id(TEXT) is a Fivetran internal unique id that helps us avoid duplicate rows in primary keyless tables.
A list of all the system columns can be found here.