Hagia Sophia is an important historic church to Orthodox Christian. St Sophia cathedrals were built in Kiev, Polotsk and Novgorod – three important cities in Rus at the time. St Sophia cathedral in the cities were rebuilt in Baroque styles except possibly in Novgorod in later years.  In Bulgarian capital Sofia I believe there’s a second oldest Bulgarian cathedral named after St. Sophia.

Here’s the original St Sophia cathedral built in Kiev in the 11th century in Byzantium style. https://en.wikipedia.org/wiki/Saint_Sophia’s_Cathedral,_Kiev