Hi,

Please refer to the module:

1. https://odoo-community.org/shop/partner-relations-1802#attr=942482

This module defines typed relations between partners (company–company, company–person, etc.).

2. https://apps.odoo.com/apps/modules/18.0/partner_contact_personal_information_page

This module helps to handle partners vs. companies.

Hope it helps.

See https://www.odoo.com/forum/help-1/multi-level-customer-structure-how-can-i-add-a-subsidiary-to-a-company-record-not-a-contact-i-need-a-grandparent-parent-child-hierarchy-for-my-customer-record-122659

You cannot have an individual that works for different companies - you need to create them twice - this is because legally they are two separate individuals. 

When they work for Company A, Company A is the company liable for their behavior and debts.
When they work for Company B, Company B is the company liable for their behavior and debts.