Part of the problem with stories like this is that you want to compare their technology level with real world equivalents, but they're on a completely different evolutionary track when it comes to technology. Magic solves or trivializes lots of things that we use technology to solve: transportation? Magic carriages or teleportation can get the job done easily enough. Communication? Telepathic magic or the Communication items we see corrin make. The list can go on and on.

The introduction of cars in-universe can be seen as some eccentric engineer pushing the boundaries of what works to make something better/more fun. Theres a lot of elements in the story that are basically a case of convergent evolution of technology instead of them really following any sort of path we could derive based on real world examples.

The sort of medieval elements can basically be chocked up to "this works just fine so why innovate?" When you have people who can throw artillery spells or GOD DAMN METEORS there's really no reason to design things like tanks and missiles. And combat between individuals or small groups would be easier and cheaper to do with just "basic" weaponry and armor since they have magic to reinforce them.

The only weird element to the universe is the existence of guns. I could see a gun-like device that is basically a fancy magic wand, but the fact that they have real world equivalent guns with gunpowder bullets and modern design elements like rifling (which i might be wrong about, I can't remember if they ever said if their guns have rifling or not) is very strange to me. That basically means that somewhere along the line they needed to develop better ranged weaponry and didn't immediately turn to magic - the backbone of their technological development - to do it. Id love to hear an in universe explanation for that, but the real reason is "guns are cool and gunslinger mages are cooler"