I have a 1-hour YouTube video I uploaded 2 years ago on my channel. I want to play the video during a YouTube live stream. Is this directly possible? Online suggestions are for OBS, but it is an overkill - I want a video already on YouTube servers to be played during a YouTube live stream.
With the setup of YouTube's new premiere feature, you can have a video file played as a live broadcast.
So in your case you can download the video you already have from YouTube servers by going into your creator studio -> videos -> click the 3 dots on the video you want and download it.
After that, upload that file as you usually would, but on the last stage there will be an option to choose how you would like viewers to see it.
You have private, unlisted, public, and finally live premiere. You're just going to want to click that and it should play live at the time you want.