Complex sentence: If the source data uses a surrogate key, the warehouse must keep track of it even though it is never used in queries or reports; it is done by creating a lookup table that contains the warehouse surrogate key and the originating key.
Split into simpler sentences: If the source data uses a surrogate key, the warehouse must keep track of it even though it is never used in queries or reports. That is done by creating a lookup table that contains the warehouse surrogate key and the originating key.