I'm looking for novels where MC transmigrates/reincarnates from modern world to ancient world and uses the modern knowledge to their advantage (creating weapons, developing technology, etc)
Eh, wouldn't put human emperor in that. He doesnt do much with modern knowledge to create modern stuff. Him creating hairpins for cousin while is modern is not what I think OP wants.
What you are looking for (I think): Ascendance of a Bookworm Bone Painting Coroner Seiun wo Kakeru Slightly outside the search range (but my personal recommendations): Assassin Farmer Tensei Shitara Slime Datta Ken Lazy Dungeon Master
Kinda half what you want: https://www.novelupdates.com/series/tensei-shite-inaka-de-slowlife-wo-okuritai/ MC recreate pasta and reversi and shogi but it don't takes a lot of chapters. This one is quite good. Really cunning and smart MC.