While not most online games are free (many premium titles thrive), free-to-play (F2P) dominates player counts and revenue. Here's why the model works.
Path of Exile (ethical monetization: only cosmetics/stash tabs), Warframe (everything earnable), Team Fortress 2 (pioneered the model), Apex Legends (fair battle pass).
Research a game's monetization before investing time. Communities like r/patientgamers often flag predatory practices.