UI Best Practices

I'm trying to figure out the best practice for creating an interactive UI. 
I'm mainly focusing on Mobile, and had some okay results with attaching an entity to a player and attaching that to the screen, it feels wonky in a number of ways and doesn't completely do what I'd like. 

Ideally I'd like to have a basic inventory system that allows for players to select an item from the UI. 

Ideally, I'm looking for some very specific guidance on this as I've gone through tutorials/ documentation multiple times but, still feel like I'm not understanding the solution.