Individual Site Permissions - Restrict External Collaborators

Hello--

For SharePoint Online, is there a way to enable some sites to be able to make their content accessible to external vendors (separate company, organization) while locking down other sites so even a site owner can't make their content accessible to external vendors, or is this a tenant level setting only?

If the external access can be set at the site level, can you please direct me to where I can learn how that is done?

Thank you,

Stephen