By default, the related products are simply other products from the same product collection.
If you would like a specific collection to be used for your product's related products, you can follow these instructions:
Create the related collection and make note of the collection's handle. The collection's handle can be seen by clicking 'Edit Website SEO' within the collection itself.
See below for an example
More on collection handles here: find the collection handle →
Edit your product and add a new tag that specifies the collection's handle:
The products in this collection should now be displayed in the Related Products section.
For more dynamic and customer targeted related products, we recommend the Recomatic Shopify app.