We don't know. But they are really elaborate and real-looking.
The bulletpoints we don't know either, but they're more like your usual bullshit rumours that know little, very specific details while others that should be more easily identifiable aren't even brought up.
It could just be that it's a world inspired by those concepts. After all, fictional settings are always influenced by real life stuff (sometimes very obviously), GRRM included.
In any case, I really don't put much trust in those words (the images, maybe).
And I too remember comments about this not being Norse-inspired or whatever (that was a big rumour some time before E3 2019 iirc).
Yes, in this IGN article you can read Martin's involvement. Miyazaki is going to make the plot and lore and Martin is going to make the mythology.
The collaboration started by Miyazaki explaining Elden Ring's main theme. With a mutual understanding of the game's concept, Miyazaki wrote the main story while Martin is responsible for the world's mythology.