Hello,
Not sure how far you reached on the BOM transfer process.
Are you planning to use Trade Preference ? Despite of that, You need to define cross plant/ plant based BOM evaluation in GTS Org Structure with the respective plants.
Once you are done with GTS configuration, you need to complete the control settings for BOM transfer in GTS plugin on ECC side. You have several options here to define how you want to control BOM transfer.
please do let me know if any further information is needed. Thanks.