Step 1

Go to Products → Collections → Blog Collection

Step 2

Search for the product you want to add.

Step 3

Now go to Products → All Products → Particular Product. And add the Blog Collection near the Collections.

Step 4

Go to Online Store → Blog Posts → Particular Blog. And give the created tag near Tags. Eg: Related--smarter-collagen

Result

By following all the above steps, we can display on the related product in the blog pages in Shopify.