For your virtual gamepad, you often have a bounding box for the controls in screen space. Detect when a touch appears inside one of those controls, you could but must not use a spatial positionening data structure for this, a simple rectangle vs. point check should do the trick as well for a limite…

I think I understand now the view matrix well. We have:

objectSpace      →      worldSpace       →      cameraSpace      →       clipSpace         →         NDC space      →       viewportSpace 
           [Model matrix]         [View matrix]         [Projection matrix]    [perspective W division]  …

if you're serious about this then u need a serious course, grab one from udemy.com is my advice, and u'll be able to ask authors any eventual questions u may have.

Until then ?