I opened a private community link. Why can't I see its games or join?

Because the community is private: only its participants see its games, people, leaderboards and history. From outside you see the name, avatar, cover, city and number of participants, with “This community is private” and a “Request to join” button.

Tap “Request to join” (you need a Playbox account; without one you are sent to sign up and brought back, then tap it again). The banner changes to “Request sent”. The owner or an admin approves it, and you get a notification. If they decline, you are not notified and can ask again.

Having the link or having played one of its games does not make you a participant. A public community has no join step at all: its games are open to everyone.