It was actually 3.5 GB for PS4 at the beginning despite having additional 256 MB DDR3 on board. 3 GB for XB1. As a side note Sony is extremely afraid to talk about these things now for PS5, it requires utmost secrecy apparently, the masses aren't ready for the disclosure.
Very relevant topics in 2022.
Well, it would be interesting because if the XB1 had more RAM available, then you‘d expect it to show in games, maybe with better textures or something. But that didn’t happen; I can‘t think of a single multiplatform game that had better textures on XB1 compared to PS4. I mean, if games on XB1 had one or two extra GB available, that should’ve been pretty significant, shouldn’t it?
Both PS4 and XB1 freed up RAM as the gen went on. They both start with more reserved for OS so that they don't get caught out on features and as the gen progresses memory optimisation and a more refined OS feature set mean they can free it for games. I think the PS3 at some point even changed where some OS data was stored. ie it decided that it wouldn't store some OS features in memory but shift it from the HDD when you press the XMB button. It slowed down responsiveness a little but offered more RAM to games. Now with super fast SSDs I think the OS footprint is fairly small and the UI pretty responsive.
Yes. This was especially evident in the PS3 gen where mandatory OS updates were even stored on the disc so you can play the game without a connection.