You may wish to add additional channels (directories) to your team at the top level. In the example Team site below, in addition to the General channel, you could add a new channel for read-only resources, separate directories, or focused resources.
You can add a channel from the teams interface by first selecting Teams on the left bar. Then click your Team's name from the main window, click the ellipses (…), and then choose Add channel from the menu.

Adding a channel also creates a file folder in SharePoint Online, which you can view by clicking on the channel, choosing Files in the ribbon, and then clicking Open in SharePoint.

If you have already created a root level folder in SharePoint under the base Documents folder, you can add a channel with the same name, and the folder will be added to your Team site.
To make any file or folder (and it’s sub-folders) read-only, follow the instructions here:
https://support.office.com/en-us/article/create-read-only-files-for-class-teams-0e7791d7-8c9c-4749-9bca-984289477988
Or, right-click on a folder in SharePoint, and select Details.

The Details pane opens on the right side.
Note: By default three groups are created: Owners, Members and Viewers. You may not be able to see the full name of the groups, but the icons will indicate which group is which (TO, TM,TV in the example below).
You can change a group's access by using the pull down (Can View), or select Stop Sharing to remove that groups access altogether.

You can also use the Grant Access field at the top to add groups or individuals. If you do not uncheck the “Notify” field, then each individual will get an invitation.
