If it is a Foreign Key reference to your products table, then set it as such, but you need to set products.id to Entity Key, if it isn’t already. This would automatically be done if your database has PK and FK defined already.
I guess it failed auto-detect, since it has ..._id in the name, but couldn’t find any references. It’s auto, not magic
If you want to show it as an integer, then set Field Type to Number and click to define formatting.
The product_id field actually isn’t the primary key, but also not a foreign key. It doesn’t point to another table in the database.
Changing it to a number just changes it from 1,876 to 1.876. But I want it as 1876. Is there another field type I can use? Either in my own database or in Metabase?