“During the Early Christian era (c. 4th–mid-8th century), the domed apse became a standard part of the church plan, and from the time of Constantine I, it was placed at the west end of the basilica (e.g., Old St. Peter’s). Between the 6th and 7th centuries the Roman branch of the Catholic Church changed the orientation of the apse to the east, as the Byzantine churches had done earlier. The apse was the most elaborately decorated part of the church, with the walls sheathed in marble and the vault ornamented with mosaic that depicted an embodiment of the godhead.”
Image and excerpted text from Brittanica Online, https://www.britannica.com/topic/apse-church-architecture