Hi Clement,
I’d like to sum up your requirement (please correct me if there is any misunderstanding or something is missing).
1. The entire SharePoint Online structure (P plan)
I have attached a diagram which might give you a basic picture of the entire SharePoint Online structure for P plan.
The SharePoint Online for P plan stands at the site collection level, which means there is only one site collection available in SharePoint Online for P plan. The top level of the entire site collection is the public facing web site. There is a default sub-site has been already created for you which is the Team Site. You can also create multiple sub-sites/pages under the top level site collection and create multiple sub-sites/pages in side of the sub-site itself. For more information please refer to SharePoint Online planning guide for Office 365 for professionals and small businesses.
2. The SharePoint permission level
Permissions enable users to access resources that they need. For example, permissions enable users to do something such as open an item in a library or create a sub site. Often, single permissions are grouped into a collection of permissions known as a “permissions level”. For example, if you want to give someone permission to read items on your site, you must also give them permission to open the page that contains the item. The Read permission level includes all the single permissions that a user has to have read items. For information, please refer to Understanding permission levels.
Thanks,
Kent Gu