Where is the code that determines order_item_id?

Hi,
I’ve got a webshop running that has products with variations. When I add the same product with the same variation, I want it to generate a new order_item_id, in stead of combining the two under the same order_item_id.

I can’t seem to find the bit of code that is responsible for generating the order_item_id.

Could anyone tell me where this bit of code is located?

https://www.ads-software.com/extend/plugins/woocommerce/