@Venkateswaran TR Thanks for reaching out. While we might need a more clarity about your exact scenario, it seems you need to start a way of syncing users and computers to Azure AD with multiple forest.
I would suggest you to start from here : https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/identity/azure-ad
as that link has all the topologies we support including multiple forest to same Azure AD single tenant sync options.
Followed by user sync options at : https://learn.microsoft.com/en-us/microsoft-365/enterprise/set-up-directory-synchronization?view=o365-worldwide
For choosing the device identities , you should start from here: https://learn.microsoft.com/en-us/azure/active-directory/devices/
We would love to understand more in detail, about your requirement, do let us know more about your question.