Hello Lora,
For "Products Related Products" addon you need to relate the products with each other. Like if you want to relate cameras of same brands, open the product > than go to the related products tab > create a group. After that go to product and select that group.
When in frontend you open any of two product, the related product will be visible.
For more information watch the attached video.
https://drive.google.com/file/d/1on2-i2N2gmDzwxTlB5iElbM1Ce-eJi7B/view
Thank You