Many of the background settings appearing in the manga and anime may be digitally rendered versions of real locations in Japan.
It is the 21st century. The society is semi-utopian, allowing high numbers of people to play mahjong, making it a major sport. People are generally good. They rarely lie and they don't cheat. One of the worse moments has been Teru Miyanaga falsely denying that she has a younger sister. A mahjong coach has slapped a student. Tragic events do occur, like the accidental deaths of Koromo Amae's parents. Families may split apart or move often due to work.
Most of the coverage of mahjong is of the All Japan High School Mahjong Tournament. The tournaments use riichi mahjong rules that don't exclude the use of supernatural powers. Several girls possess supernatural powers while most of the media covering mahjong are unaware of such powers.
Human sexuality is unusual, at least in some parts of Japan. Many girls have romantic feelings for each other well into their high school days. Underwear is very rarely worn and many skirts have high hemlines.