I've been wondering this for years, OP. I guess I always assumed they all had a little screen on the machines with the lyrics. All this bluetooth and YouTube stuff seems like it would make for a really janky karaoke experience.
Does anyone remember those cable channels that were karaoke channels? We used to love those for afterparties in our 20s. Only people with the fancy cable subscriptions seemed to have them. Now I'm wondering if they still exist!