Either you are using BC's Manufacturing Module, or you aren't.
1)
You can use Business Central's Subcontracting feature, which comes with Manufacturing module, which will satisfy all your requirements at once. Some good videos available out there, but I'll tag Microsoft's official documentation
learn.microsoft.com/.../production-how-to-subcontract-manufacturing
However,
1. To specially monitor the stock, create a handful of subcontracting locations in Business Central. Whenever you want to see certain stock, check on this location
2. You simply would receive the PO, as described in the link, which will auto-book your capacity ledger
2)
If you are looking/expecting to use Subcontracting without the manufacturing module, I'd suggest use "purchase order" as your so called "sub-contracting" order, you may also use certain dimensions for additional tagging. As mentioned, you can manually create a handful of subcontracting locations, to keep track of which item is sitting where. In the same way, you can "post" your PO, to mark the sub-contracting goods receipt & pay those subcontractors/vendors through general/usual OOTB AP module
3)
If none of the above suffice, you may also want to check out some 3rd party apps available on Microsoft AppSource, try a few for free and see which one suits you
