What is the difference between Fajr prayer and Subh prayer, which of them is the obligatory (Fard) and which is the Sunnah, and are Sunnah prayers performed in congregation?

The Fajr prayer and the Subh prayer are two names for one obligatory prayer. As for the Sunnah of Fajr, it consists of two rak'ahs, performed after the Fajr adhan and before the Fajr prayer. It is a confirmed Sunnah and not obligatory. A person performs it individually, either in the mosque or at home. The Prophet, peace and blessings be upon him, was diligent in performing them and made them very light.