But if that happened, it was already done before our oldest manuscripts of the Gospel of John. Perhaps some early scribe listed an example of those "many other miraculous signs" after the end of the book (borrowing a story from Luke 5:1-11) and the next scribe copied that section as if it were part of the text. This chapter includes the reinstatement of Peter, who had denied Jesus a few chapters earlier. If it is an addition, that would help make sense of John 21:24 which speaks of the author of the book in the third person. So, the only evidence that this passage was added by a scribe is the internal evidence of the text itself. John 21 seems like it was tacked on to an already finished book, but there are no surviving manuscripts that omit John 21.
But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. John 20 ends with what looks like a closing statement: If you see the Bible as a book written by a perfect God and transmitted by fallible humans, then you must discard this passage as a human invention, which is a shame because it teaches a nice lesson. Most modern translations include this passage but label it as a later addition. One scribe even stuck this story into the book of Luke. Interestingly, we have manuscripts that insert this story at different points in the Gospel of John. It may have simply been a part of the oral tradition about Jesus that was added to the margin of a manuscript by a scribe and inserted into the text by a later scribe. There's no obvious reason for the insertion of this story. It is the source of the iconic phrase "Let he who is without sin cast the first stone." Although it is a charming story about grace and forgiveness, the textual evidence suggests that this passage was not in the original version of the Gospel of John. This passage contains the story of the woman caught in adultery.
(I also discussed the ending of Mark here.) 2. The addition of this ending brought Mark in line with the other Gospels and smoothed over this inconsistency. It served as a record of the changes that had already been made to the stories about Jesus. The abrupt and unimpressive ending of Mark may have been a source of embarrassment for the church. Why they changed itīy the time this passage was added, the other Gospels with their post-resurrection appearances and ascension accounts were well-known throughout the early church. So, they added some post-resurrection appearance stories and a commission from Jesus calling people to be baptized, speak in tongues, heal people and handle deadly snakes without being harmed. This may have been all there was to the story when the Gospel of Mark was written, but when church leaders were copying this book more than a century after it was written, this abrupt ending must not have seemed right to them.
Our most reliable early manuscripts of the Gospel of Mark end with Mark 16:8, which says that some women discovered the empty tomb of Jesus, but never mentioned it to anyone. But in some cases, entire verses or passages were inserted by the scribes to bring the text in line with their theological views and to bolster their position in the debates of the day. Most of those changes were minor mistakes like spelling, word order, skipping a line or just a simple misreading. We now know, based on comparisons of the surviving manuscripts from the history of the Church, that those early copyists made changes to the text. The same Christians that were preserving and copying the texts that became the New Testament were also embroiled in several theological debates.
For the first 300 years, that work was done by literate church leaders rather than by a class of professional scribes and monks as was the case later on. The printing press was invented in 1439, so for the first 1400 years of Christianity all of the books were copied by hand.