Global Security Group is a scope for the group and is set for created groups: https://docs.microsoft.com/en-us/windows/security/identity-protection/access-control/active-directory-security-groups#group-scope

You need to set the Sharing settings to 'Existing Guests'. This setting allows sharing only with guests who are already in your directory. These guests may exist in your directory because they previously accepted sharing invitations or because they were manually added.
Reference:
https://docs.microsoft.com/en-us/sharepoint/turn-external-sharing-on-or-off

Your organization might use Windows Server FCI to identify documents with personally identifiable information (PII) such as social security numbers, and then classify the document by setting the Personally Identifiable Information property to High, Moderate, Low, Public, or Not PII based on the type and number of occurrences of PII found in the document. In Office 365, you can create a DLP policy that identifies documents that have that property set to specific values, such as High and Medium, and then takes an action such as blocking access to those files.
Before you can use a Windows Server FCI property or other property in a DLP policy, you need to create a managed property in the SharePoint admin center.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/protect-documents-that-have-fci-or-other-properties