Live update frontend

How can I update a webpage live, without the user refreshing the page or doing any thing?

Example: Audience is awaiting a live video stream to start, but until we go live, we show them a promotion video. When our artist is ready, we reveal the live video.

I imagine that I must use fetch-api to replace content (video url). But how do I do that in a smart way? Will fetch need to be run every second, and wont that use many resources. Thanks so much!