Thank you for the detailed information, @ahmed elmosalamy.
I'm here to take care of your concern today and guide you on how to properly build an assembly in QuickBooks Desktop (QBDT).
First, create an assembly item, and add the materials or items it composes under the Bill of Materials section. Once completed, you're now ready to build an assembly.
To create an inventory assembly type of item:
- Go to the Lists menu, then select Item List.

- Click on the drop-down arrow, then New.

- Choose Inventory Assembly as type, then add the name and the income account.
- Add the materials under the Bill of Materials field.

- Hit OK once finished.
After that, you're now ready to build an assembly:
- From the Inventory tab, tap on Build Assemblies.

- Choose the item on the Assembly Item drop-down.
- Add the quantity you want to build.

- Click on Build & Close.
For additional guides about building and creating assemblies in QBDT, you may check out these articles:
In case you need to reorder components from your vendor, you can send a purchase order to them. Then, track the items you receive.
Please let me know how else I can help you with your assembly item. I'm always here to help. Have a good day!