Page permissions are set at the category level. All pages in a category will inherit its category's permissions. If you want to make an individual page accessible to certain people (inside our outside your organization/Tettra team) without changing the category permissions, try link sharing.
Categories are set to visible to all your teammates by default. You can also set categories to private, only inviting certain people to view them.
To change the permissions on a category, go to a categories settings by navigating to a category and clicking the 'edit' button in the top right.
In the category settings, you can switch "Who should be able to view this category" to "only certain people".
You can then select the people you want to be able to view that category.
n.b. if you have a community account, all categories will be set to public.