Everything I learned from the 2 hour 20th Anniversary video and from going through all of the developer's commentary nodes.
20th Anniversary video here: https://youtu.be/YCjNT9qGjh4
Developer's Commentary nodes: https://youtube.com/playlist?list=PLRDhcp_8c7uDXb2nT67c9CrCGihW0e02C
See their website about it here: https://www.half-life.com/en/halflife2/20th
0:00 - Summary
1:23 - Ending at the start
3:29 - Fake stuff
4:16 - Cool easter-eggs
6:00 - Hidden story
8:08 - Characters
8:42 - Graphics
9:55 - Commentary on the dev commentary