Let me understand the process:

Site 1:

Makes product A, product A can be sold directly to consumers or sent to Site B where it is used as a component.

Site 2: Makes product B, product B is made from product A

Product B should have a BoM that has product A as a component. Product A should have its own BoM.

You need to define a custom route for product B that does the following:

If there is not enough product A at Site 2, then create a transfer order from site 1

If there is not enough product A at site 1, then create a manufacturing order for product A.

Depending on your knowledge, you may be able to figure this out yourself.

However, it involves specific configurations across multiple areas and needs to be setup according to your other requirements (e.g. Is product A make-to-order or make-to-stock etc..)

Learn more about routes/rules here:

https://www.youtube.com/watch?v=9OoviT3uYnk

There are also many other resources online