What did Jesus do during the "unknown" — or "lost" or "silent" — years, between the ages of 12 and 30?

Here's a Wikipedia article on this gap in the Biblical record, which seems to invite speculation:

Some Arthurian legends hold that Jesus traveled to Britain as a boy, lived at Priddy in the Mendips, and built the first wattle cabin at Glastonbury. William Blake's early 19th century poem And did those feet in ancient time was inspired by the story of Jesus traveling to Britain. ...

In 1908 Levi H. Dowling published the Aquarian Gospel of Jesus the Christ which he claimed was channeled to him by a supernatural being called "Akashic Records" as the true story of the life of Jesus, including "the 'lost' eighteen years silent in the New Testament." The narrative follows the young Jesus across India, Tibet, Persia, Assyria, Greece and Egypt....

The usual story is that he "simply lived in Galilee during that period."

Christians have generally taken the statement in Mark 6:3 referring to Jesus as "Is not this the carpenter...?" as an indication that prior to the age of thirty Jesus had been working in the trade as a carpenter. The tone of the passage leading to the question "Is not this the carpenter?" suggests familiarity with Jesus within the area, reinforcing that he had been generally seen as a carpenter in the gospel account prior to the start of his ministry. Matthew 13:55 poses the question as "Is not this the carpenter's son?" suggesting that the profession Tektōn had been a family business and Jesus was engaged in it before starting his preaching and ministry in the gospel accounts.

But was he a good carpenter? He left the business to take up itinerant preaching, and at that, he was divine. But how was his carpentry? What was the back-and-forth between Joseph and Jesus in the carpentry shop? The gap invites speculation, and I picture scenes in which Joseph critiques his son's work, and Jesus gives elegant, elaborate explanations, replete with parables, and Joseph gets irritated at the fine craftsmanship of his son's words, and says it would help if he'd apply a sliver of that craftsmanship to the job at hand, and Jesus thinks I'll show him, I don't need these tools and this wood, the words are my tools and I will craft men.

We see echoes of that old father-and-son debate in the seemingly spontaneous quip to Peter and Andrew in Matthew 4:18-20:

As he walked by the Sea of Galilee, he saw two brothers, Simon who is called Peter and Andrew his brother, casting a net into the sea; for they were fishermen. And he said to them, “Follow me, and I will make you fishers of men.” Immediately they left their nets and followed him.