Hi @Taha ,
based on your provided information I don't see a reason to setup a separated Active Directory just for the external users.
Most of our customers are organizing the external users in their on-premises AD in dedicated OUs.
This way the external users are really easy to identify and managed.
A challenge managing external users in your own AD, it doesn't matter if in your central AD or an additional AD, is to manage the user accounts if a user left the external organization. Over time this might cause orphaned user accounts in your AD.
One way to get this managed could be to monitor the last login date of external users. If a user's last login was x days before, for instance 180 days, deactivate the user, after additional 180 days delete the external user. Just an example.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten