It's very rare to find a system design explanation of a certain company that presented towards a beginner. So all Reddit architecture explanation from Steve Huffman that presented in Udacity was indeed gold for me.
Sadly I can't find it anywhere. The only thing I found is this complaint in HackerNews and CS 253 Miror in Google Sites. I am very happy to find the mirror. But sadly it didn't contain all the materials. Finding it throughout the official Udacity site currently impossible, all the link to CS 253 was redirected to its homepage. Even it's available on youtube, there is no playlist for it, so it's scattered all the way around.
In the meantime, I find one of my favorite programmer, Peter Norvig. I looked into the URL, there was a pattern:
I tried to play with it, changed the course code to "Course 253"
AHA!, all the old CS 253 course are still there, but the link is hidden. Quickly I download all the videos to keep it locally :). I read the license of udacity video is CC Attribution-NonCommercial-NoDerivs 3.0. So I might upload those videos to archive.org in case the "hidden" link was taken down completely.
PS: you can take the "Intro to Backend" course but some material from CS253 is missing.