SharePoint Site Plan

Hi guys, a bit new to SharePoint so I need some help for SharePoint Site architecture for multiple companies including file sharing and access.

Background:

Our company consists of one company with 8 "sub-companies" all with:

1. different companies
   • MAIN COMPANY
     • Company abc
     • Company xyz
     • Company klm etc.
2. different domains
   • email address removed for privacy reasons
   • email address removed for privacy reasons
   • email address removed for privacy reasons
3. different tenants on Azure AD
4. different employees having rights to different files

We are moving from a on-prem server to a hybrid server (365 & on-prem).  Some of our applications (accounting, payroll, etc.) will be kept on the on-premise server and all files (Excel, Word, etc.) will be moved to  365, hence the SharePoint sites.

What we want:

We want to build a SharePoint site (Home Page) for the MAIN COMPANY and subsites for each company (Company abc, xzy, klm, etc.). 

The MAIN COMPANY will house:

1. HR Documents
   • Policies and Procedures
   • Leave forms
   • etc.
2. IT Documents
   • Policies and Procedures
   • Other forms
   • etc.
3. Other Documents

The sub sites for each company (Company abc, xzy, klm, etc.) should have sub folders (file sharing for collaboration) for different departments within each company:

1. Finance
2. Marketing
3. Operations, etc.

These should have different access permissions based on the employee level (Manager vs Clerk vs EXCO)

We have the following architectural structure in mind:

We would like:

1. for all employees across all companies regardless of employee level to access the HOME PAGE as well as the content (Policies and Procedures, Training Material, etc.)
2. create links to the subsites for each company to access files and folders to collaborate on (with certain access permissions) 

We would then have:

1. HR document sharing (collaboration) in a separate site for obvious reasons
2. IT Document sharing for the IT Department only - Optional

Our Challenges:

Our challenges sits with the permissions to grant as we have different domains.  We would then create the MAIN COMPANY in one tenant (Azure AD) and create guest users for each company to allow file sharing across companies.

We also have growth in mind (adding new companies under the MAIN COMPANY)

Assistance needed:

1. Is this the correct approach?
   • Any other suggestions would be welcomed
2. Can we get some videos on this on how to create something like this?