It consists of several simple apps instead of widgets so that you can freely place only what you need on the home screen. Installation is required for each, but the "manner mode" application that creates volume save data is required first. At a minimum, you also need a "volume on" app that restores volume.
This "headphone mode" app turns off everything except the media volume and the volume of the voice during a call, and turns on the vibration function.
Please note that the volume changes depending on the connection / disconnection of headphones as a function originally found on smartphones, regardless of the function of this application.

Details: http://hinakin.main.jp/phone/mannermode/index.htm

You need permission to access "Notification ringing restriction" ("Silent mode" depending on the Android version).