Short answers, pulled from the story.
The Basilica of Junius Bassus was built in 331, at the start of Junius Bassus's consulship. An inscription recorded this founding date. Bassus had previously served as praetorian prefect for over a decade before his promotion to consul prompted the construction.
The basilica stood on the Esquiline Hill in Rome, on an estate owned by Junius Bassus. The site is now occupied by the Pontifical Oriental Institute, near the Piazza di Santa Maria Maggiore.
The opus sectile mosaics are marble panels made by cutting stone into precise shapes to form images. The surviving panels depict lions attacking calves, the abduction of Hylas, and a portrait of Junius Bassus in his consular robes on a chariot. They were removed from the site in 1903 and are now held in the Capitoline Museums and the Museo Nazionale Romano.
In the fifth century, a Romanized Gothic nobleman named Valila acquired the property and donated the basilica to the bishop, after which it was dedicated to Saint Andrew and used as a Christian church. The pagan mosaics and the original inscription honoring Junius Bassus were preserved inside the converted church.
Valila was a Romanized Gothic member of Rome's urban elite who acquired the basilica property in the fifth century, possibly through a marriage alliance with the Bassus family. He donated the structure to the bishop for use as a Christian church dedicated to Saint Andrew, while retaining the original dedication inscription to Junius Bassus.
Art historian Gregor Kalas has proposed that Christians reinterpreted the pagan imagery rather than removing it. The panel depicting the abduction of Hylas may have been understood as an allegory for Rome's transition from paganism to Christianity. A homily from the time of Pope Gregory I also shows the site's pagan past remained part of how the church was understood.