Custom integration product data syntax to connect events and catalog

I am implementing a custom integration and there is a question I can't seem to find the answer anywhere. When I track a "Viewed Product", what do i need to pass to the event? and what is the schema of this data in order for it to assign the view to the correct product catalog?


If you look at the sample data Klaviyo offers, catalog and events seems unrelated. But I can't wrap my head around this. There should be a connection between the events and the product catalog.

 
Sample Product (take a closer look at the id)

Sample events
 

From what I've explored in the dashboards, there isn't a place where I can check which of my catalog products are being added to cart and whatnot. 

Any help is appreciated!