Why did the mobile phone suddenly receive a short message, showing that UnionPay has been credited and the funds are not low?

Nothing more than three situations. First, the bank made a mistake and the data was wrong. Second, someone really transferred you a sum of money. As you said, the amount is quite large, but I didn't tell you in advance. Third, someone plays a prank or tries to cheat you. They will send you a message with special software, and then display the bank number, but the former is harmless, and the latter wants to play your game.