The reason the Romans had better cement was not because we suddenly lost the knowledge to make it, it is because we lost access to the specific materials for their form of cement, and so ended up losing the "recipe" for it.
I dunno about the other things where "the ancients did it better", but this is definitely not the case for the Roman Concrete.
Let me quote the most important part related to this from the article I linked a bit above.
Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability.
So, it really seems like a case of losing the recipe. Especially because...
As a result of these successful tests, the team is working to commercialize this modified cement material.
“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” says Masic.
Through the extended functional lifespan and the development of lighter-weight concrete forms, he hopes that these efforts could help reduce the environmental impact of cement production, which currently accounts for about 8 percent of global greenhouse gas emissions.
So like, we definitely have had access to plenty of the material to make the Lime Clasts that make the Roman Concrete durable.
We just... Lost the knowledge, and it took us a huge amount of time to figure out what was missing.
... Though apparently we hadn't figured it out for so many years mainly because of incompetence tbh. Like...
“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”
How did nobody think this until now? Like... Seriously? Nobody tried experimenting with the Lime Clasts until just now and instead just assumed that they were there due to low quality control? o.0
Feels like incompetence on the part of the researchers to me, but it doesn't change the fact that the recipe wasn't lost due to losing access to the materials.
Dude, I'm on my phone, but most of these stories involve magic with a classic 4 element structure. I could make a fusion bomb with the water element alone. People assume nukes are complicated. They aren't. The addition of magic allows the breaking of physics in ways that will hurt your brain.
Don't have time to do the math, but yeah, real world physics applied with magic to get past certain limitations equals nukes.
Generally speaking, the way magic and physics interacts isn't perfect. Otherwise it becomes too easy to break the setting (and many things plain and simply don't work if you try respecting physics too much).
Magic is magic, it doesn't need to follow physics.
Now if you are griping about non magical settings as well, saltpeter plus sulfur plus coal equal boom. Give me time to experiment, in a society without gunpowder or magic, I'll have cannons. Once I have cannons, I win. Grapeshot against knight's win.
You have to know that saltpeter + sulfur + coal creates explosives (far from a common knowledge), and you have to find ways to obtain those materials in large enough quantities, figure out ways to safely experiment with them (don't wanna die in your own experiments, after all), and you have to figure out how to make a cannon that withstands the explosion.
You also need to figure out ways to properly preserve the ingredients, as I imagine too much humidity/dryness can alter them.
Then, you need to find ways to make your cannon mobile (well, you can put them on forts at least, to avoid the mobile part) and easy enough to aim to be usable... And well, you'll still need knights to protect your cannons (since they'll probably be unable to rapid-fire), and you'll need to make sure to not hit allies with friendly fire with your extremely unstable weapon.
... Doesn't seem like as much of an easy plan as you're suggesting. Especially if you're reborn as commoner.
The average modern dude, just understanding the concept of the assembly line, can transform society. So the Manga condenses it down to a shorter time frame? They take dramatic license with the specifics. Big deal.
Uhn... How will you incorporate a functional assembly line without something akin to actual machines that require the assembly line?
What is your blacksmith assembly line? What is your seamstress assembly line? What is your woodcutting assembly line?
This... Doesn't seem helpful in the slightest.
Second the creation magic lets him create anything he can think of
... Can't he make a carriage moved by oxygen or something? I mean... If anything goes, then you can probably do something better than the Hummer.
Or rather... Good old super robots? Make them move together with the pilot's movements (magically, of course) so as to not need to create any complex control system, and make them powered up by oxygen or whatever. This way you only need to know how to properly imagine the joints and you're good to go.