The kids only know each other as commenters on a video podcast which just stitched together trending TikTok videos. Since the only consumers of such content are kids whose parents ban them from social media, they got to know each other in that podcast's comments. When the podcast was taken down the kids, who all add each other on Spotify, see the other playlists of the other kids (one named "Chat here") and they go to the podcast in that list and chat there.