Shortly after the Ascension of Jesus, the small Christian community (Luke says there were about 120 of them) gathered and decided there should be a replacement for Judas as one of the Twelve. They cast lots, and Matthias was chosen.
The practice of casting lots went back to ancient Jewish tradition. People sometimes turned to their priests to determine God’s will. The priest would cast sacred lots, “Urim and Thummim.” (It’s not known exactly what these pieces looked like – they may have been precious stones with some type of characters on them.) Magic and superstition were forbidden to Israelites, but casting lots was not considered superstitious.
By using this method to choose Judas’ replacement, the early Christians showed their Jewish roots.
After his election, Matthias is never mentioned in the New Testament. Although there are legends, little is known about him with any certitude.
Nothing is known about what became of the structure itself that was called “The Twelve.” This structure is last mentioned in the sixth chapter of the Acts of the Apostles (The book has 28 chapters). The structure called “The Twelve” is never heard of again in Acts, or in later New Testament writings.
I disagree with the statement about "The Twelve" never being mentioned again, because in a recent reading from Revelation twleve courses through Jerusalem were marked by the names of the Twleve apostles. Hmmmm.