As per my knowledge if we define that only users inside specific security group can share with external users, as follow:-
then this will be applied to SharePoint sites + onedrive personal sites. now inside our organization we want everyone to share files with external users inside their one-drives, while only allow users inside the above security group to share the sites with external users.. so is this possible? or the above settings will get applied to sharepoint and onedrive?
I just tested this, and with the setting applied it would not allow me to share externally from SharePoint or OneDrive from a user not in the security group. As soon as I turned the setting off again, it allowed me to share once again. So at first glance it sure looks like this is a settings that is either on or off across both SharePoint and OneDrive.
This particular setting is only available at the organization level today and so is working as expected. The policies here will apply to content stored in OneDrive and in SharePoint. There is no way to configure separate lists of allowed users today. If you have any questions, feel free to let me know! Thanks!