Martial King's Retired Life - Chapter 12Book 16:

In a distant future, the empires of mankind span the galaxy, and glorious Earth has devolved into a peripheral backwater.

In Shanjing city in the Asian region, Wang Zheng’s dreams of becoming a mech pilot are crushed when his college entrance exam genetic score turns out a pathetic twenty eight, barely above an animal.

To make things worse, people get the impression he attempted suicide after being rejected by the campus beauty.

Then the closest thing to a family he has, the old man in the book store across the road, goes missing, leaving him only a mysterious birthday present.

This novel is also known as Master Fu's Mysterious Darling Wife

She is the The Young Miss of Military family Shen, her face is peerless, her temperament is cold, and she is her family’s treasure, but isn’t accepted as one’s child by her mother;

He is The Sole Heir to the top giant Fu Family, his power is unparalleled, indifferent high-born, the captain of the mysterious military force, and the youngest commander-in-chief.

Because of her grandmother’s wishes, she married him at lightning speed. She thought that it was a marriage that she needed, but she didn’t expect to be greeted with his extreme pampering.

- Description from Novelupdates

He wants indoor plumbing. They think he’s conquering the world.Arthur was a genius engineer on Earth, capable of optimizing global logistics and automating entire factories. But he lacked one thing: common sense. Now, reincarnated as Arthur von Pendelton, the six-year-old son of a powerful Duke in a medieval fantasy world, he faces his greatest challenge yet: Drafts.Armed with the T.E.C.H. Interface (Trans-Epochal Construction Heuristic), Arthur can analyze magic like code and build modern technology from scratch. His goals are simple: Double-glazed windows, hot showers, and a toaster that doesn't burn the bread.But there is a problem. The people of this world don't understand engineering.He builds a high-pressure boiler for a bath; his father thinks it’s a “Dragon-Breath Siege Weapon.”He creates a hydroponic garden; the local church declares him the God of Harvest.He ignores a Princess's flirting to analyze the thread count of her dress; she thinks he is playing “hard to get.”Follow Arthur as he accidentally industrializes a kingdom, terrifies his enemies with home appliances, and remains completely oblivious to the fact that he is becoming the most powerful man in history.All he wanted was a warm bath.

To save a doomed fantasy world, I killed the main characters, then destroyed the game world, and fell into another timeline as the academy's top seat and rival to the protagonist.Now, I'm trying to save the world and steer it towards a better ending…But I'm the final boss.