How to declare Header BOM and Components in Export Declaration to Customs Authorities

[Former Member]

Hello,
We have situation that BOM already exploded in the Sales Order and also Delivery, when Proforma is created in ECC with reference to delivery, generates an export declaration in GTS along with BOM Header and Components materials.

ECC Proforma and GTS export declaration looks as follows:

BOM-Header (not physically shipped, virtual item)
- Component-1 (physically shipped)
- Component-2 (physically shipped)
- Component 3 (physically shipped)

I am not taking about compliance management (license determination) but talking purely about how to declare export declaration to customs authorities.
I have not transferred BOM to GTS but only transferred as individual materials (component and header materials).

I have 3 questions:
1. Do I really need to transfer BOM to GTS just for Customs Management Export Declaration purpose (not interested in Preference management & Compliance management )
2. When export declaration transmitted to German Customs, how do I communicate to customs Header and Components separately in the export declaration
3. In this situation how aggregation functionality works

If the answer to my question #1 - NO need to transfer BOM to GTS then
For the 2nd question, I found in blogs to use BOP ID in the export declaration item level - So, in ECC i can generate and populate BOP ID via User Exit during proforma transfer to GTS. So, what would be BOP ID for Header material and Component materials when transmitted to customs for them to identify.
Is BOP ID is to be populated with some unique number OR High Level Item (BOM Header material Item No.) for all the component materials?

3rd question, how aggregation works as BOM Header Material and components

Inputs appreciated please.

Thanks