The Settings button is displayed in the player toolbar if it is enabled in the builder and there are settings that can be displayed. If there are no settings, the Menu button in the player is disabled.
The settings window can be configured in the Elements / Settings window
You can choose the order and value of each listing (quality, audio tracks, subtitles etc). Pressing the Menu button in the player will open this window.
If you want to customize the design of this element in the builder, you must enable some settings. For example, specify several quality options in