To fix entities onto the camera such that they stay within view no matter where the user looks, you can attach those entities as a child of the camera. Use cases might be a heads-up display (HUD).
<a-entity camera look-controls> <a-entity geometry="primitive: plane; height: 0.2; width: 0.2" position="0 0 -1" material="color: gray; opacity: 0.5"></a-entity> </a-entity>
Note that you should use HUDs sparingly as they cause irritation and eye strain in VR. Consider integrating menus into the fabric of the world itself. If you do create a HUD, make sure that the HUD is more in the center of the field of view such that the user does not have to strain their eyes to read it.