Security rights in documents folder in Sharepoint online site

Hello,

We have created an public M365 Online SharePoint site.

We want to share this site with external users. That works.

Then in this SharePoint site you have the menu 'Documents'.

In this documents folder we wants to create some subfolders.

But each subfolders has it's one rights. Is like in Active Directory.

Example : you have a purchase and sales folder. Then you have a purchase and sales security group.
People in the purchase group can only access the purchase folder. And people in the sales group can only access the Sales folder. If some of the Sales group click on the Purchase folder then you get the message 'Not authorized'.

So external user email address removed for privacy reasons can access the site. In the SharePoint site we want to put that user in the 'Purchase' security group so only access to the Purchase folder.
External user email address removed for privacy reasons can access the site. This user in the 'Sales' security group. So only access to 'Sales' folder. Is this possible to configure ?

If you how can I do it ? Links to manual ? 

Thanks for your respons.