Hi there,
You can set the permissions on folders by clicking the 3 dots next to the folder and selecting Manage Access. In the next screen select Grant Access and enter the email address of the person, or enter the Office 365 Group, who should have access. Select the access type and probably you'll want to deselect the Notify People Checkbox.
If you need to maintain read-only access, Teams and private channels might not be the best solution for you. Both are built on the principle of "equal access for all members", and in some cases, this can be extended to every user in the organization.
Teams channel folder permissions https://learn.microsoft.com/en-us/answers/questions/301339/teams-channel-folder-permissions.html
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