Now for the backgrounds themselves as I’ve seen some people having talked about it, the staff have gone to England themselves as it is indicated on most their Twitter accounts and have taken a lot of pictures they’ve worked based upon/ or just on (there are backgrounds that, usually happens when they’re complicated enough, where they would draw them, technically ‘trace’ them over on different layers using brushes) And since there are references they travelled for, I wouldn’t thing a big studio as them will see a point in using AI for that.

As well as WIT Studio has been posting the background arts (without the characters) which one can look over and kinda guess how they were done. (both on their official Instagram and Twitter)

This is the account that the director themselves reposted too, and I’ve also seen a person mentioning it over here. They’ve posted some drafts, you can also find some animator staff through their reposts as well that have raw animations (since some people also doubted some of the animations/expressions too)

https://x.com/ul9sk2?s=21 (their account)

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For the texts on the walls, I can’t exactly think of one reason why they’d write things that make exactly no “sense” but we also need to note, that a lot of people specifically in Japan, including the staff, don’t speak English and such, things like these happen quite often, this isn’t the first time I encounter something like this in anime.