Hello,
So your quastion is " How to track products when using subcontracting feature"?

follow this steps:

1. Create your product, define BoM , add subcontractors.

2. add subcontractor to purchase tab in product template.

3. Create new subcontracting location in Inventory -> Configuration -> scroll down to Locations.

Note: You have to activate multi step route feature in Inventory Settings and subcontracting in Manufacturing settings.

4. Go to vendor Contact and specify location

Note: you have to activate developer mode in order to see these fields

5. Create purchase order for subcontracted product

6. Confirm purchase order

Note: it will create two transfers 1. to resupply BoM components to this vendor 2. to recieve final product from this vendor

7. Going to Inventory reports, you can filter based on subcontracting locations

Note: this is v16 views but it keeps the same behaviour as previous versions

So, you can select specific subcontractor on purchase order OR if you replenish it the system will choose the top vendor in Purchase tab of Product template.

Hope this will help.