Hi,
As you said, link building helps in keyword optimization of your app in App Store Optimization (ASO) but it is a costly affair to get good backlinks, links from not so good sites can we even harm your rankings.
As a basic starter, we can start with optimizing your description, posting relevant screenshots in perfect order, sharing the app store link in your social network, Quality keyword-rich content, reading reviews, optimizing logos and optimizing for other secondary keywords as well.