Simple and small free app that can always adjust the volume and is useful for movies, music.

You can adjust the volume of music, movies, and apps, and change from advanced options to phone call and speakerphone volume.

Click the Advanced Features button on the 5th row.
The first row "Sound" button allows you to select several types of volume.
The 2nd line "Mute button" can be muted by clicking the volume button on the screen once or twice.
The 4th line "Show and Hide" button can hide or show the buttons on the screen and show the scroll bar. You can also hide or show volume notifications on the side or bottom. There is a function to show buttons according to the screen rotation status or music playback status.
The "Volume Lock" option, second from the bottom, is a function that prevents the volume from rising above the set level.
Other than that, "App Appearance" sets the button color or font of the app.
The 6th line notification setting is a function to decorate the top notification bar.
The 7th line icon change is a function to change the volume icon.

The green button on the notification bar is the hide function.
The speaker button indicates the volume and can be muted by double-clicking in the advanced options.
If you press and hold the speaker button, you can activate the function that disappears in the advanced options.
You can resize the icon in the notification bar.

How to turn on/off button on screen with send intent in app like tasker:
1. Intent to hide the app
Action
com.oksoft.hide
Extra
hide:1
Package
com.alwaysvolumebutton
Class
com.alwaysvolumebutton.OnLock_BroadcastReceiver
Target
Broadcast Receiver

2. The intent to show the app.
Action
com.oksoft.hide
Extra
hide:2
Package
com.alwaysvolumebutton
Class
com.alwaysvolumebutton.OnLock_BroadcastReceiver
Target
Broadcast Receiver

For always visible rotate button, put com.alwaysrotate instead of com.alwaysvolumebutton.
For other always visible apps, just change the package name.