The 16-year-old boy, who cannot be identified for legal reasons, has pleaded guilty to planning a terrorist act over the course of more than a month in 2024.
In a sentence hearing on Tuesday, the teenager gave evidence about how he was gradually exposed to increasing levels of Islamic extremism online.
He first saw extremist material on TikTok in 2024 when aged 14 and became interested by accounts of conquests in previous centuries.
"It was historical ... the time of the Crusaders, empires a long time ago," the 16-year-old said.
Impacts of the aftermath of 9/11 and Islamic State propaganda were increasingly shown to him and the videos also became more violent, the teenager said.
While he started to become more radicalised, he began communicating with other people on encrypted app Telegram about his desire to learn about committing terrorism.
"Did you like talking to them?" the boy's barrister Michael Valentin asked.
"Yes, I found it interesting ... (they were) a new group of people, I was trying to relate to them," the 16-year-old said.
The teenager then began researching how to target the Sydney Opera House with a bombing on one of its busiest days - the running of the Sydney Marathon.
The iconic landmark serves as the finish line for the globally significant race, making it an ideal target for maximum impact on human life, the 16-year-old said.
The idea for choosing the marathon came from watching videos about the 2013 Boston Marathon attack, where three people were killed and more than 500 were injured by two bombs planted at the finish line.
"I am picking the location, places where lots of people and tourists are out," the offender agreed he said to someone online.
"I will test and then do the operation, inshallah (if God wills)."
A message reading "the country I am living in helped America evade Iraq and Afghanistan and killed many Muslims" was his justification for planning the attack, he said.
Despite his detailed planning, the 16-year-old said he was only "50:50" about whether he would actually carry out a bombing.
He spoke of having a "little voice" in his head trying to persuade him against committing terrorism.
"The consequences and it would have been bad for my family too," the teen said the "little voice" would say to him.
Ultimately, the young person said he disavowed everything IS stood for and held no extreme views.
"I'm deeply sorry for the things I did," the boy told NSW Supreme Court Justice Stephen Campbell on Tuesday.
"Please show me some remorse."
His lawyer urged a non-custodial sentence to enable the teenager to continue his deradicalisation in the community.
Having been on bail under strict home detention since November 2024, the teen has been allowed to attend events under his parents' supervision, including the Vivid festival and the Royal Easter Show.
"Clearly, we have before you a young man who ... has seen the errors of his ways," Mr Valentin said.
"Thankfully he is now out of the rabbit hole."
Crown prosecutor Sophie Callan SC acknowledged the "candid" evidence of the teenager but said there is no other appropriate possibility than a term in custody.
"There remains a level of naivety in expressions of insight ... that would leave Your Honour with a degree of caution," Ms Callan said.
Justice Campbell will deliver his sentence judgement on October 29.
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