Pairs often confused
aussi or donc in French?
Aussi means "also" AFTER the verb; AT THE START of a sentence it means donc, and it inverts the subject.
Why
"La bibliothèque est aussi fermée" adds. "Aussi la bibliothèque est-elle fermée" concludes: the hyphen and the resumed pronoun are required. Without the inversion the sentence reads as a misplaced addition. It is a C1 turn; at B2, donc does the same job safely.
aussi
Le musée ferme le lundi ; la bibliothèque est aussi fermée ce jour-là.
The museum closes on Mondays; the library is also closed that day.
donc
Les crédits n'ont pas été votés ; le projet a donc été reporté.
The funds were not voted; the project was therefore postponed.