Hagia Sophia was once a Christian church and once a mosque. Now it's a museum. What's in the museum are bits and pieces of the churches and mosques from different time periods -- over 1400 years, many things have changed.
Modern era changes are perhaps easiest to see. One big change occurrd in the 1800's, when the sultan commissioned two Swiss brothers to renovate the mosque. Most notably, they added the roccoco chandeliers and the large, circular calligraphy panels. They also documented, with the Sultan's approval, as many of the original Christian mosaics as they could find and then plastered over them again to preserve them.
Calligraphy is a major form of art in the Islamic world. And these panels were created by one of the most prominent calligraphers -- Kadiasker Mustafa Izzet Efendi. (I believe that the teardrop shaped collection of characters is his signature.)
Curiously, this Efendi fellow's work has been part of the Washington Monument since 1884.