Japanese culture: Is it true adult adoption is common in Japan?

True adoption is not that common.

However, if you read the Japanese literature with automatic translation, you will find many more references to "adopted"(養子) than this.

This derives from the way surnames are decided when getting married in Japan. In Japan, when people legally marry, they are required to unify their surnames to one of them: around 90% of people unify their surname as a couple using the male surname, while the rest adopt the female surname. This unification with the female surname when getting married is a kind of slang for 'adopted'.

What happens if a parent refuses to let their child be transgender? What happens if the parent tries their hardest not to allow their child to be trans, like flushing every bottle of their trans child's HRT down the toilet?

This is due to the fact that Japanese civil law prior to WW2 did not allow one to choose to adopt a woman's surname when they married. If he really wanted to choose a woman's surname, he had to be adopted into a woman's family and married at the same time.

This custom died out after WW2, but the word 'adopted' remained in the language. It has actually been used as the title of a manga.

There are two main current examples of this being done: firstly, because a family with a very large estate but no heirs wants to create a new person to manage the estate. The other is for gay people who want to marry in Japan, where same-sex marriage is currently not possible, to be legally recognised as a family. Neither is very common, considering the overall percentage of the population.

To a flat Earther, what's wrong with the idea that gravity is simply a force inherent to space which operates only in one dimension? Why do they go further and try to deny gravity rather than just saying it's different than physicists claim?