Hi Jean,
Please note that a user is only able to see courses in hidden categories if they have the capability moodle/category:visibility allowed. This capability only works if the role is assigned in the system context i.e. the role is a global role.
Basically, it's not possible to allow students to see any courses in hidden categories unless their role is global. If a user is assigned the global role of student then they will have this role in every course on the site. Is this what you require?
Roles and permissions
Visible courses in hidden categories
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