Tumsī and tuṣbiḥu are imperfect verbs with an implicit “she” as subject. Abītu has an implicit “I.” The conjunction places the two clauses in an immediate contrast.
Fawqa ẓahri ḥashiyyah and fawqa sarāti adham both use fawqa as an adverbial expression in an iḍāfah construction. Repeating the same spatial expression makes the contrast unusually precise.