Need help on how App can change Storefront looking

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I am new to App development and working on an app to tag the Product, then show the tagged products as related products on the product page. 

Tagging part seems easy. 

But I can not find any docs regarding how to show tagged products in storefront. 

I found this blog:

But it does not too many details.   Please point to me where I can learn related topics. 

Thanks a lot.