archives, surname
宬 centers on a place where records are kept: an archive or repository. The surname meaning is a separate, unrelated use.
宬 combines 宀 (roof, building) with 成, which likely contributes the sound. The character suggests a building where completed records are stored.
Under a roof 宀, completed 成 documents are kept safe: that's an archive.
For セイ, imagine a sage (セイ) carefully storing scrolls in the archive: sage -> セイ, and the wise person guards the records.