Either find a way to use the roles / capabilities system or make your own version of the app: https://docs.moodle.org/dev/Moodle_Mobile_Customization etc. Or see / raise a Moodle Tracker to have an enhancement for 'stealth meta courses'. Also look at the documentation: https://docs.moodle.org/35/en/Course_meta_link.
Themes
Hiding courses in Moodle app
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