I live in Turkey. I have been on several domestic flights, including one to Ankara. I have also been on two international flights, one to London/Heathrow and the other to Zurich, Switzerland.
I have never had to clear Customs or fill in any Customs form. I think it is not needed. I have gone through immigration (passport control and baggage claim). I see people asking about clearing Customs or whether they need to go through Customs when they are in transit.
Who needs to go through Customs, and when?
What happens if you want to buy from Duty Free? In some countries, Duty Free is not in the international area. For example, when you arrive, it is after passport control, so you are in the country. How can it be tax free?
I am a student and researcher. I am sure that, in Japan, you have to complete a form about Customs.