As per the following diagram, rebeladmin.com domain has two domain controllers:
The FSMO role holder is running a domain controller based on Windows Server 2012 R2. The domain and forest functional levels currently operate in Windows Server 2012 R2. A new domain controller with Windows Server 2016 will be introduced and it will be the new FSMO role holder for the domain. Once the FSMO role migration is complete, the domain controller running Windows Server 2012 R2 will be decommissioned. After that, the forest and domain function levels are raised to Windows Server 2016.
In the demonstration, REBEL-WIN-DC01 is the domain controller with Windows Server 2012 R2 and REBEL-SDC01 is the domain controller with Windows Server 2016.
When you introduce new domain controllers to existing infrastructure, it is recommended to introduce to the forest root level first and then go to the domain tree levels.