MTL - From the Dungeon, Slaughter the Demon Apostle - Chapter 668 the key to this

A scientific genius named Thomas Harrier was transmigrated to an imperial prince body after a car accident that took his life. He found out that he is in a world similar to our own with a timeline akin to early twentieth century. The body that he is occupying was the imperial prince of a failing empire who suddenly rose to power after a successful assassination attempt of his father. Unaware that the attack is just the beginning of what is to come. He faces many challenges throughout. Will Thomas Harrier be able to live his second life in peace while running an empire and getting it back to its former glory? Or will he succumb into a conspiracy lurking within the country's borders?

Expect some:

Cold war era techs.

Super battleship with advanced electronics, propulsion, and fire control systems?
Helicopters?
Missiles?
Modern medicines and equipment?
AC-130?
A nuke?
Challenger or Abrams or Leopard?
F-4 Phantom or MIG-21?
Colored TV's.

Everything!!!

PS. Cover not mine, I just found it on google.

Athena Grant,a human,dies in a car accident and becomes Eira Liora,an elf. Transported to the barren and cursed island of Luinmir, a once-thriving land now abandoned.A year ago,their King announced that the island,Luinmir would no longer be under the empire's control. It was well known that the land was barren and poverty stricken...She was abandoned both by her Kingdom and her Father.She wakes up one day with a system.“So what if they were abandoned and deemed cursed,she would show them all the highest peak of knowledge and development. The island's core is broken...fine,she'll get it replaced but this time,the island would become hers to keep,she would make sure of that.”

Ren Takahashi, a shy, hoodie-wearing tech genius, hides behind the online alias GhostHood — a quiet legend in underground hackathons. Despite appearing average in school, he dreams of becoming a real-life Iron Man — not the hero, but the builder of world-changing tech.Enter Aoi Hoshizaki, a bold, gal-style girl with a love for code, a messy room, and a mysterious spark. When she moves in next door and discovers Ren's secret, their world begins to change — from quiet club meetings to national hackathons, late-night debugging sessions to secret rooftop conversations.What starts as teamwork grows into something deeper, more complicated, and far more electric than either expected. Together, they aim for Japan’s most elite tech university — and face rivalries, rumors, and the whisper of love in every line of code.

They were born with magic, raised in chaos, and trained in sarcasm.Now the royal kids are all grown up—and the world isn’t ready.Between family feuds, resurrected villains, and a five-year-old firebender with an attitude, the palace is basically a magical daycare with crowns. But when revenge hits home and the kids take charge, things get wild, weird, and a little too flammable.Royal duties? Nah.Sibling wars? Always.Saving the kingdom? Accidental.Welcome to the most dysfunctional, dangerously magical, and absolutely hilarious royal family you’ll ever meet.