New design AD root forest TLD

Question

Good day everyone,

We are looking to deploy on premise brand new AD on organization that is in growth and has the following:

1. it has a initial public domain that currently holds email service on microsoft 365. company01.com (this was the first company and its currently working and has the majority of users)
2. it has 2 new public domains each for separate companys ie company01.com and company03.com
3. it has 1 new public domain for the holding group. group.com

We would consider in the future to deploy Exchange Email server to work in kind of hybrid landscape if that does make sense.

We would consider in the future to extend the active directory services to azure cloud if does that makes sense.

What would be the proper or best practice to set the initial root forest domain in such escenario.

Thanks in advance