You can find an introduction to the topic of content packages here: What is a content package?
If you want individual decks to be visible only to certain users, you must add the deck to a content package. To do this, edit your deck. Switch to the library and select the desired deck.
You can assign one or more content packages. Click in the input field to display all available content packages. Select the desired content packages with the checkbox.
Save your change and check the visibility in the app.