I'm looking for something where the mc uses their knowledge of some modern field (math/science/tech) to find new ways to cultivate or create new magic spells. Heck, even build tech. But it's important for the author to know his shit to some degree so the mc isn't just pulling things that make no sense out of his ass (I don't want the classic mc is a hacker who touches a computer and it solves all his problems without explaining the hacking to at least a semi realistic degree). What's hit closest to this on novelupdates is RTW, but not necessarily the kingdom building part; more about the using science part. Mushoku Tensei kinda did this but broke my suspension of disbelief because his intuitions were extremely obvious, and didn't use any scientific knowledge. 'look, we can have electricity to shock people' is not really surprising to anyone who has experienced a static shock, and it's not interesting to read about because it's not an inventive use of magic or science. A fanfic which is very close to what I want in terms of using knowledge of some modern field is The Arithmancer by White Squirrel. Most important about this is that the math used isn't complete bullshit, and is a unique and clever way to apply math to magic. For example, I'd like to read a story about the person who invented the stellar transformations technique in ST. TLDR; Want mc who has math/science/tech knowledge and uses it in a new world to become powerful.
Ouroboros Record actually had the mc come up with a way to use alchemy to speed up the decomposition of material and utilize it to create rich, fertile ground.
The dao of magic on rrl. more than a hundred mid sized chapters and regular updates. the main character looks at cultivation through the lense of a scientist rather than a cultivator. He made a construct (jet pack like thing) out of energy, enabling him to fly way before he should have been able to.
Lord xueying has some well done science fantacy once you get past Ark five. also the same auther as stellar transformations.
The essence of cultivation - mc is a mage, teleported to a cultivation world. I'm [Biologist] in a Fantasy World (i don't remember the title, maybe is little diffrent) - mc is a biologist caugh up in a hero summonig The 3rd law of cultivation ... - mc is a scientist in a cultivation world. everything above is on royalroad
Release that Witch That's really the entire premise, using modern technology and witches to build his own kingdom.
https://www.royalroad.com/fiction/2826/a-heros-war - (note: on indefinite hiatus, author works at a hospital, so doesn't have time to write for now) 2 POV: the "hero" and the MC. The MC wants to uplift this magic world so the people can properly battle the zombie hoards, but to do so, he needs good steel. So he's making a big push to jump start an industrial revolution along with integrating the use of magic. The hero is off doing his own thing on the other side of the continent.