Makes me wonder what Ash was in his past life. Maybe a really sincere and diligent public servant?
He could have been any office worker with a decent public education. In an isekai-style fanfic, the MC remarked to himself that his Japanese high school education probably made him the most knowledgeable person in a room full of military generals. Certainly there are some manga in which the MC applies to join the Merchant's Guild, and finds that the 'very difficult' exam included elementary arithmetic.
By the time you leave high school, you already know that other continents exist and that citrus fruits can prevent scurvy. You might know both integral and differential calculus, and the scientific names of beings animalculus. In short, in matters vegetable, animal, and mineral, you are the very model of the modern major general.
cough uh, sorry.
You would know the history of your country, including the notable downfall of foolish kings. You would have learned that weights and measures should be standardized and that cobblestone roads need to be crowned to let water run to the sides and not pool in the middle. You might have heard of the hub-and-spoke model of distributing goods and mail, and the historical importance of a professional civil service (which can keep a nation running even if the king is insane). You might even have learned some of the basics of celestial navigation, which can revolutionize both naval power and international/intercontinental trade. There's also germ theory and public sanitation, including how to develop an urban sewage system that can prevent cholera outbreaks.
For that matter, knowing the principles behind the Rocket Stove would be a lifesaver. Its design, once refined, can burn wood much more efficiently while greatly reducing smoke and smoke-related respiratory illness. Efficient wood use can be of benefit to the poor, who struggle to heat their dwellings, and also reduce deforestation. Also, if it is less expensive to heat water, people can safeguard their drinking water by boiling it, greatly reducing disease. This design is not taught about in schools, but anyone might happen upon the design by viewing enough YouTube.