SharePoint Sharing with External

I am not sure how the policy is implemeted in your organiztion tenant.

If it is conditional policy then Entra admin can exclude the domain or users in CA policy.

If this in in Sharepoint Sharing settings, then on tenant we have to modify this and enable external sharing and limit domain to limited domains. and also in each site we can manage similar settings.

Also it good have a different site and configure external sharing only on that site so data on different site will be secured.

Also everyone in Tenant can not share to new external users as it is limited to Role "Guest Inviter". User must have above role to share with new guests. But if guests(external users) already in tenant they can share .

how to control on tenant : https://learn.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off

How to control on site : https://learn.microsoft.com/en-us/sharepoint/change-external-sharing-site