@RyoukoKonpaku To be honest, Japan and another isekai-ed nation in this series are basically wildcards (there are 3 isekai-ed nations, but the first one that was isekai-ed is a tier 1 civilization), but Japan is still the most technologically advanced. And about the advanced ancient magic civilization all I can say it's that if you play Stellaris, or at least know how it works that lost civilization is basically a mid-late game crisis event
And as a little add-on, Japan basically spawned in, what that world calls as, an uncivilized area and pretty far away from the real great powers, so that's why things are looking underdeveloped
P.S: I don't consider what I said as a spoiler because I didn't tell whose nations I'm talking about aside from Japan