configuration: Odoo Community V13 on own server; I already enabled the checkbox "Share products between multi-companies" and "Share contacts between multi-companies", so I need to share only Bills of materials):

1. Settings -> Activate the developers mode

2. Settings -> Technical -> Security -> Record rules 

3. Search "mrp.bom" as "Object". You will find a line which looks like 

mrp_bom multi-company ...  Bill of material ... ['|',('company_id','child_of',[user.company_id.id]),('company_id','=',False)]

Click on this line.

4. On the top right there will be a green text "On". Put mouse over - it will change to "Switch off". Click on it