When I searched videos about pub/sub, I came across this video.
Turns out the guy who's making this video is a famous competitive programming person.
He made an entire playlist for Basics of system design, a must learning topic for anyone who wants a deep understanding of how the actual web works.
Why I wanted to share this with you is because, the videos are explained in with simple words and nice pronunciation, which makes easy to understand.
Hope this will be helpful!!!!