"Without imports" means resources. Do Japan have all the mines, deposits, and resources to build them without imports? That's why I said without imports. If they can substitute the lacking materials with another world's materials, then it's all good.
So far they've filled their agricultural and food imports thanks to Quatrayne. They've got all their oil need from Qulia. What makes you think they can't find other mineral resources? Do you think that other world lacks elements and minerals to make gunpowder, guns and bombs when the other civilizations have muskets and cannons? Or you think Iron, Carbon, Steel, Sulphur, Salpeter, etc are hard to find resources? Do you think they can't make something as simple as nitric acid, glycerine, etc for more modern explosives? And Silicon for electronics is not some magical element, do you know Japan is still one of the top exporters of 99% purity silicon used for electronics, so what makes you think they can't refine more as long as they can mine or import high purity silica rock?
https://oec.world/en/profile/bilateral-product/silicon-dioxide/reporter/jpn
Japanese corporations like High Purity Silicon Corporation and Shin-etsu refine silicon for wafers in their facilities in Japan.
Also Japan still has a stable mining industry and natural resources, it's not a damn barren island like you may think. Not to mention all those untapped resources at the bottom of the ocean and islands around it like the Senkakus and Kurils, why do you think they have territorial disputes with China and Russia over them? Obviosly except for oil, mining anything on land is easier than in the sea, but when push comes to shove they have alternatives.
https://en.wikipedia.org/wiki/Mining_in_Japan
Most of the people in this thread who swear Japan is doomed without help from China and Murrica may need to read and research more what Japan can manufacture and what kind of resources they still have.
In our world most of the 1st world countries have exploited the raws resources of 3rd world countries, what makes Japan exploiting resources from less civilized countries impossible? And just like mentioned before they established trade with Quatayne and Qulia, the ambassadors from other countries friendly with Japan also trade with them as shown in early chapters. Seems like no one got the blatant message from the first few chapters with getting all their food and oil needs so quickly solved is handwaving from the author that Japan got pretty much all their needs solved. Even Altaras is mentioned to be known to have magic stone mines and the reason for Parpaldia's subjugation and now an ally of Japan. What makes you think they don't have other types of mines. They established diplomatic relations with Mu which is a WWI level civ. What's so different from a WWI civ to today's civ material & resource wise? It's not like Silicon, Rare Earth Metals and some chemicals used in modern life didn't exist before and suddenly and magically appeared, we just found a use for them or how to refine or produce them from resources already present in our planet.
This is a manga, there are details that they will gloss over due to lack of space and pacing. Geez like the fate of the foreigners & embassies was glossed over but still explained in a couple of lines in ch. 19 of manga, yet still people kept asking and questioning what happened to them like if that small explanation was not enough or as if it was never explained. And even the LN it's just a couple of paragraphs. Or the chat between the ambassadors about diplomatic relations and trade with Japan. More information can be inferred the few details given and don't need pages and pages of extensive exposition. Can't people use their brains or do they need every effing thing spelled for them? What do you need as proof? That they present you a list of each and every country that trades with Japan and a list of every damn element of the periodic table by name that they have and sell to Japan especifically?
If you want to know more:
The land beyond the gate that Japan helped defend from the Demon Lord in ch 18-24 has plenty of all kinds of resources they even sent expeditions for it, although that has not been covered in the manga and only in a Gaiden volume of the LN. As for current/near future affairs: at the end of the war with Parpaldia, as reparations Japan demands a certain amount of gold that even though compared to Parpaldia's GDP it is not ridiculous but still a substantial hit if paid in one installment or over the years, Japan in the same document gives them an alternative in a permanent mining concession of a certain Parpaldia territory. Kaios chose the concession because to their civlization standards and knowledge there were no valuable minerals there and didn't understand why Japan would want to have mines there. They don't mention what kind, but can you guess what kind of resources would be mined there that's not valuable to a less developed civ but valuable to an advanced civ like Japan? Or do you require to be spelled out?
If they were in desperate need of resources or they lacked resources like WWII Germany, they'd ration the resources and even cut down its use for certain sectors. But then why would Japan start to openly export technology approved by the government like Cars, Cellphones and phones, TVs and all kind of electronics and appliances , Power generators, Cell, radio & TV towers and transmission equipment, etc to many friendly countries bringing a new economic boom in the background during all this time? If they lacked resources why would they so openly sell trains, build roads, railroads and bridges in other countries like Quatrayne and Qulia? send construction equipment to Altaras? If they lacked resources, why would they launch rockets and reconstruct the GPS network and spy satellites all while they were fighting Parpaldia? Oh yeah, because it wasn't explicitly talked about in the manga... "if I can't see it, it didn't happen mentality"?