Click on the 'Hide/Show' button ( / ) on the Taskbar to hide or show 'Memory cleanup', 'Bluetooth', 'Sound' buttons, etc.
To pin an app, go to the app's button in 'Start menu' ( ) and right click on the app. Select the 'Pin to the Taskbar' option.
To unpin the app, right-click the closed pinned app on the Taskbar and select 'Unpin'.
NOTE: Dragging an app from desktop to Taskbar creates a shortcut of the app but doesn't pin it.
To turn on 'Back Button', you need to go to 'Settings > Experimental features > Show back button on the Taskbar' and turn on the feature.