PDA is a PDA or mobile business digital device, that is, a computer's peripheral assistant, which is rich in functions and easy to use, and can meet most of your daily needs, such as reading, games, dictionaries, learning, taking notes, watching movies and so on.
APP, according to your first question, may refer to Xueyi platform, which is a mobile phone application platform developed by Shanghai Xueyi Computer Technology Co., Ltd. (a large part of domestic/cottage machines use this platform), that is, STEP platform.