The Bible has no record of what Jesus did after the age of 12 until 30, what happened during this time?
This is often termed the missing 18 years of Jesus. Many believer Jesus became an explorer who traveled to far off lands like India and Tibet and spent his time learning of other religions. They try to fill in his history of where he lived and was educated that is supposedly missing in the Bible to account for his great powers and miracles. That he learned this Spiritual knowledge by secret teachings he discovered as an initiate trained in occult knowledge.
The Scripture has a completely different view than the mystics. What is assumed as missing years is actually used not missing at all only silent until the time he is to be revealed as the Messiah the Son of God.
We are told of Jesus when he was very young And the Child grew and became strong in spirit, filled with wisdom; and the grace of God was upon Him (Luke 2:40).
The last record we have is when he was 12, Luke 2:41-47 His parents went to Jerusalem every year at the Feast of the Passover. And when He was twelve years old, they went up to Jerusalem according to the custom of the feast When they had finished the days, as they returned, the Boy Jesus lingered behind in Jerusalem. ... they returned to Jerusalem, seeking Him. Now so it was that after three days they found Him in the temple, sitting in the midst of the teachers, both listening to them and asking them questions. And all who heard Him were astonished at His understanding and answers. Jesus did not have to travel and study to get his knowledge, he already possessed it. As we can see at this young age he was already contending with the learned Rabbis on what the Scripture said and they were in awe of his knowledge.
How? He was born with it, because He was God who became man . Luke 2:11: For there is born to you this day in the city of David a Savior, who is Christ the Lord. He was Christ when he was born, he didn't become a Christ or have the Christ spirit to come upon him. He was Yahweh, the Lord who incarnated among the people that He gave His laws to and prophesied by the words of their own prophets (Isa.9:6). Jesus was specifically taught by His Father unlike any other person as he developed (Isaiah 50:4-7).
After the incident in the temple we find the Scripture state Luke 2:51-52: Then He went down with them and came to Nazareth, and was subject to them, but His mother kept all these things in her heart. And Jesus increased in wisdom and stature, and in favor with God and men. He didn't go travel seeking wisdom he had it and he stayed with his mother.
Mark 6:2-3: He began to teach in the synagogue. And many hearing Him were astonished, saying, Where did this Man get these things? And what wisdom is this which is given to Him, that such mighty works are performed by His hands! Is this not the carpenter, the Son of Mary, and brother of James, Joses, Judas, and Simon? And are not His sisters here with us? And they were offended at Him. The peoples argument is that they knew him and his family, and could not understand how it is possible to be so learned and have the power of God when he grew up among them. This proves he did not have the time to go off and study other religions but devoted his life to the Hebrew scriptures.
Luke 4:22: So all bore witness to Him, and marveled at the
gracious words which proceeded out of His mouth. And they said, Is this
not Joseph's son?
What Jesus said and taught was quoted from, and based on the Old Testament, not from any other religion or philosophers teaching. He never quoted any other prophet except his own, which makes this theory unlikely that He learned elsewhere. He led a quiet life learning the word until the time was for him to be revealed