worldbuilding headcanon; In the interest of not making people read a single two-mile wall of text (and instead two separate one-mile walls of text!), I'm going to split this up into ancient history and... semi-modern? Actually recorded? History sections, mainly because I have so much Sinjoh stuff in my brain that I need to make it its own headcanon. Consider this homework for the Tohjoh one.
Also, point of order: for clarity's sake, I will be referring to ancient Sinnoh as Sinnoh and not Hisui because I don't want to have to flop between them, and it makes more sense to call it Sinnoh when discussing the Sinjoh Ruins.
To start us off, the earliest version of the Sinjoh peoples were a nomadic tribe that traveled alongside migratory pokémon herds (swinub line), which they largely relied upon as their main means of sustenance. Their range spanned from central Sinnoh to Northern Johto. To be clear, we're talking ancient ancient. Largely before written history ancient. (Unown notwithstanding, but we'll get there.) At this time, Sinnoh and Johto were still directly connected by a strip of land.
Historically and archeologically speaking, it's unclear how and what exactly happened that these two locations were cut off from one another. Some blame a steadily rising sea level during that particular era and claim that the cut-off was a slow one, while others believe it was a single catastrophic event, such as the combination of extreme seismic activity and a tsunami that permanently flooded the plain on which these original Sinjoh people traversed, effectively cutting their tribe in half. There's not a lot of hard evidence for either, but the former theory is believed to have the more solid foundation as the existence of the Sinjoh Ruins would imply that the two halves of this culture were, at one point, able to come together to build the structure. (If anything, it's plausible that both are true.)
Regardless, this single tribe was inevitably divided. Officially speaking, the two groups that formed as a result were known as the Celesticans in Sinnoh and Ecruteans in Johto, the latter of which settled in the area now known as the Ruins of Alph. Which is the next subject of discussion.
Both the Ruins of Alph and Solaceon Ruins were constructed around the same time, specifically when archeologists have theorized this "cut-off" was occurring. Part of this is the result of the two halves of this culture, faced with no longer being able to live their nomadic lifestyle, simply being forced to create more permanent settlements. (Solaceon Town no longer has any standing above-ground structures, but it did have them at one point, as archeologists have been able to uncover. Which has generally lead experts to believe that it was the equivalent Celestican )
So... those weird underground parts and the unown, though. Researchers studying the Unown have recently discovered that they can communicate with one another across some remarkably long distances. Between regions, even.
The underground structures of these two ruins were built to serve a different purpose, as implied by the inscription in the Solaceon Ruins: All lives touch other lives to create something anew and alive. These two structures, utilizing the Unown, were meant to connect the two halves of the Sinjoh tribe when their separation became inevitable. The Sinjoh Ruins are theorized to be something of a transmission center, but there's not any hard evidence of that being the case as of yet. (If it is true, it likely also served some secondary purpose as a place of worship, as the Ecruteans were cut off from the shared holy sites of Spear Pillar and other locations in Sinnoh.)
Now, bear in mind, this is primarily a theory. Although researchers have been able to prove Unown's transmission abilities, there's not really anything definitive to suggest that they were used in that way. And the histories of the Celestican and Ecrutean peoples, as far as historians have been able to recollect, would suggest that they certainly were not living in parallel in that way.
We won't get into the decline of the Celesticans because it's not really the point here. But the Ecruteans, on the other hand?
Like with a lot of the overall history here, exactly what caused the Ecrutean people to move away from the Ruins of Alph isn't entirely clear. We do know that they left, thanks to the inscriptions in the underground portion: We humans must learn to walk in harmony with them. We depart for their sakes. (this translation being the one from Crystal, because I think it reads better). Paired with the image of Ho-Oh that accompanies it, several theories have been tossed around. All we really know is that the rise in worship of Ho-Oh led the Ecruteans to establish what would eventually become Ecruteak City in parallel with their abandonment of the Ruins of Alph. Nobody really knows why. The most popular theory is that the declining population of Unown (for unkown reasons, though some theorize that it may have been a result of the rise in worship of Ho-Oh over the Sinnoh Creation Trio) led them to simply leave and move their settlements slightly further north, where the land was ultimately better anyway. That's... about it. It's ultimately one of the big historical mysteries of the Johto region, and one that nobody really has a clear answer to.
But, the fact remains, we know the Ecruteans moved a bit further north, built the Bell/Tin Tower and Brass Towers, and ultimately integrated with the population of peoples from neighboring Kanto to form the current Tohjoh culture. Whiiiiiiiich is the subject we'll get into for the next headcanon!
