You can Delegate an Azure DNS subdomain instead of Creating a separate Azure AD domain to separate Guest users.
You can use the Azure portal to delegate a DNS subdomain. For example, if you own the contoso.com domain, you may delegate a subdomain called engineering to another separate zone that you can administer separately from the contoso.com zone.
Here is a link for a detailed description of the process that you must follow.
Delegate an Azure DNS subdomain
https://learn.microsoft.com/en-us/azure/dns/delegate-subdomain
Federating multiple, top-level domains with Azure AD requires some additional configuration that is not required when federating with one top-level domain. When a domain is federated with Azure AD, several properties are set on the domain in Azure.
Properties of an Azure Active Directory B2B collaboration user
https://learn.microsoft.com/en-us/azure/active-directory/external-identities/user-properties
Hope this resolves your Query!!
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