Welcome to the world of Daiten Daichi, the setting of the SeitenTouji series. It is a world seemingly after death, with strange rules, a surprisingly dark history, and all kinds of other systems that will be described here.
Geography
Daiten Daichi is geographically a mirror of Earth, except that many forms of animal life seem to not exist due to lack of human migration and agriculture, resulting in a very different type of environment than modern Earth, with native fauna and flora dominating. Additionally, human settlements are few and far between.
The part of Daiten Daichi where Teiki and co. reside (hereby referred to as Daiten Saitama) is lush and green, with flowing rivers surrounded by mountains. Winters and summers are mild, and the world as a whole doesn't change very much. It is the equivalent of Saitama Prefecture, Japan.
The inhabitants of the Village of the Androids, the Factory, the Cherry Village, and the Brotherhood typically identify as being inhabitants of Daiten Daichi, with their place of residence coming a close second. This is primarily due to the sparceness of the population, the distance between settlements, and the overall lack of any other unifying identity, as there are no nations in this part of Daiten Daichi.
Politics
Daiten Saitama's towns and village form city-states of a sort, where each settlement or group of settlements self-governs. Due to the lack of living population, there are no real cities, though large towns are common throughout the world. In the past when death rates within the 13-30 age group were higher, however, it seems that there were impressive civilizations... though those have mostly faded to rubble - their inhabitants either Lost or Divine Souls.
In the part of Daiten Daichi where Teiki and co. reside, there are some notable forms of government - the Road of Machinery and the factories form a cooperative unit in association with the Village of the Androids. The two governments are essentially socialist, with utilities and other benefits all being free in exchange for the inhabitants putting in their fair share of labor, construction, maintenance, and innovation.
The Cherry Village has a Mayor that oversees the town as well as an elected village council. The Brotherhood is technically a personal dictatorship, though there are so few inhabitants that it functions more like a co-op.
Culture
Over its long history, Daiten Daichi has seen many different cultures, stemming from youths being picked from all across the globe regardless of language and cultural heritage. The fact that they are youths (13-30 years) is important, as they lack the resources and expertise to build society on their own - at least nowadays. In the far past, when many more in that age group came to Daiten Daichi, there may have been large civilizations... though those are mostly lost to time. In Daiten Saitama, the primary language is English due to its use as a lingua franca for business.
It is only recently, with the advent of the industrial revolution, that Daiten Daichi has begun to re-emerge into a proper bed for civilization. Of course, this change has been horribly slow because your average 20 year old doesn't know how to build much of anything from absolute scratch, but knowledge and expertise has accumulated over time, and with skilled minds like Taiga and Yutaro, natural resource mining and processing into goods has begun anew.
Of course, given that the driving force behind everything is the power of imagination and the mind, individuals can do whatever they want to within the limitations of their abilities, and that power was used to bootstrap the current state of technology in Daiten Daichi.
Souls
Daiten Daichi is a world of mysteries - one of strange occurrences and supernatural phenomena. However, its size and scope are well known.
Daiten Daichi has existed for a very long time - after all, it is a form of afterlife. All of the inhabitants are male, and died between the ages of 13 and 30, possessing nothing but their own memories. Those who died without enough of their body intact to remain living reincarnate as disembodied Free Souls, while those with bodies or, rather, those with enough of their bodies to survive, reincarnate as such, with their missing limbs remaining missing. Notably, the selection was indiscriminate - every male human within those age bounds reincarnated. In addition, the location of reincarnation is completely random - someone who died in Scandinavia may reincarnate in the deserts of Africa. The reincarnation areas are, however, well known, and were heavily monitored in the past. Nowadays, they are occasionally monitored, but it depends on the region of Daiten Daichi.
After many years, the inhabitants of Daiten Daichi who find peace tend to become Divine Souls, while those who are unable to become Lost Souls. Divine Souls and Lost Souls can retain physical bodies if they have them, but the structure of their presence within Daiten Daichi changes. It is exceedingly rare to encounter Lost Souls who maintain a body, as most Lost Souls became such due to the death of their physical bodies.
Souls of all types, regardless of whether or not they have physical bodies, are capable of thought and have basic senses as well as standard memory. It is by communicating with Free Souls that Taiga and co. construct bodies for them. Free Souls are capable of inhabiting artificial bodies, but they are not bound to those bodies and can be forcefully removed from them. Weapons such as the Adamantine Memory Blade are capable of manipulating Free Souls who have chosen inorganic bodies to inhabit.
Some inhabitants, such as Taro, are capable of cooperating with Souls to attack, as Souls produce large amounts of Spirit Energy, the force driving the vast majority of the supernatural abilities in Daiten Daichi, and which is often transmuted into danmaku.
It is of particular note that humans are not the only creatures to reincarnate in Daiten Daichi, but given the situation with Gen'eimaru, it is probable that such events are rare and only happen in exceptional cases. There are no female humans in Daiten Daichi, though there exist females of other species. As for genetic modification? Perhaps someday someone will be born in Daiten Daichi, but that's a story for the future...