Closed doesn't mean undiscoverable. The biggest closed group chat I'm in is the Telegram channel for a podcast I listen to, and it's plugged in every episode. Discord has issues, but group chats there are still mostly fed by public activity like subreddits or YouTube channels.

Closed just means you can't open a page and read the full history like a forum. It doesn't mean it's only knowable to people currently inside and people they invite.