The main thing to notice is that the design is symmetrical by having an equal amount of smelters on both sides, uses the same speed of belt throughout the build and is fed it's resources equally by using one Splitter to even split an input line 50/50 to both the top and bottom. To expand upon things further, if you build something that is evenly balanced side to side or top to bottom and also feed resources as equally as possible then you will inherently have a balanced output without having to use any lane balancers or other tricks.
It's that simple! Smelters are a great way to demonstrate this, but these principles can be applied throughout an entire factory. You can achieve Inherent Balance by simply building and feeding things evenly. What Do I Mean By Inherent Balance?It's actually a pretty simple concept but may not be something you initially think about when you start the game (I know I didn't) because you just want resources and may not care how you get them.