How do you play a game with others on Twitch?

I have OBS, Amazon Prime, Steam, Discord and of course an account on Twitch. (no affiliate) But now I have almost no idea about the technology and only managed it with great effort and help have to get the camera and the sound running and make a few streams. I've seen that people in their stream sometimes play scribble or something like that with their viewers. Can someone explain to me without technical terms what I have to do?

where do you get e.g. The game? Will this be installed on the hard drive? Do I have to post anything on OBS or Twitch?

You play Scribbleio in the browser

In ob's you select the browser for capturing

There are many tutorials on YouTube