The earliest surviving assignment of a mission field to Andrew comes from Origen, quoted by Eusebius, who sends him to Scythia, the lands north of the Black Sea. On that thread a vast later structure was built. Medieval tradition made him the founder of the see of Byzantium, and the Church of Constantinople claimed him as its apostle in conscious parallel to Rome's claim on Peter, a claim with obvious weight in the rivalry between the two churches. From it descends his standing as patron of the Greek and the Russian churches.
His martyrdom belongs to the apocryphal Acts of Andrew, a work of perhaps the later second century, which has him crucified at Patras in Achaea. The detail that his cross was X-shaped, the crux decussata now called the saltire or Saint Andrew's cross, is not in that text. It is a Western elaboration of the Middle Ages, and the diagonal cross became his emblem only then.
Relics said to be his travelled to Constantinople, then to Amalfi and to Scotland, whose national flag carries his saltire and whose patron he is, alongside Greece, Ukraine, Russia, Romania and Cyprus. The historical man behind all of this remains the fisherman of the Gospel lists; the rest is the work the churches did with a name.