Multiple questions on stream obs, single pc setup.

I'm going to try to help you out.

 

In Windows, having a game in full screen will automatically hide every other window behind it on a single monitor. The only window that Windows will display on top of a full-screen application/game is the Task Manager. For the best experience, you would need a 2nd monitor if you want to have the game in full screen and still view and interact with chat.

An alternative is to join your twitch channel with your mobile device (phone, tablet, etc) and you can view and interact with your chat that way.

Adding an overlay is a vast topic. There are probably hundreds of youtube videos on creating and adding overlays

/r/Twitch Thread