Hey there @tbelgrave! So in most use cases you’d be correct to use UMG for the exclusive UI elements, but for the game elements it’d definitely be best to either use Sprites(Paper2D) or Meshes for anything you’re going to be using lighting or physics. So in the example you showed, UMG would be great for the keyboard and text portion, but for the images you’d have the choice between billboards or meshes.
Scalability on mobile is definitely an important concern, light physics one just a handful of actors is usually fine for even the cheaper tiers of smartphones, lighting has to be relatively simple however as depending on how many objects there are in your scene it can get expensive for the economy tiers of phones if your scenes are both movable and complex.
Here’s the docs for mobile lighting and what is supported: