Us humans barely understand our own technology that's older than a few 100 years, the Antikythera mechanism, Stonehenge, Inca's and Egyptians all had very basic tools and crude calculations compared to ours and we barely understand both what they had to say and what they did with it, most of it still being a mystery.
The Antikythera mechanism is pretty well understood and many of the wheels have been mapped to celestial phenomena (like the movement of planets during the times of the ancient Greeks), cultural events (like the Olympics), and even a simple calendar. A really neat feature is a part that predicts the timing of solar eclipses in Europe based upon patterns the Greeks observed in the past.
Stonehenge is a bit more complicated because the culture who built it didn't leave much in terms of written records or explanations of their rituals, although it is plainly obvious that the observation of equinoxes and solstices were integral parts of its purpose along with perhaps other celestial bodies including the Moon. That isn't even unique to Britain as such solar observatories have been found in other cultures on completely different continents. The Mayan observatories which did the same sort of thing even have some written records that describe their use and oddly follow the patterns of Venus as a major component.
In contrast crazy stuff like the Cloaca Maxima, something that is as great of a masterpiece of engineering compared to the pyramids and any of the other ancient "wonders of the world", is extremely well documented including instructions that have been preserved that were essentially a maintenance manual. It is very understood as to its purpose, and it is even recorded as to who made it in the first place (not just a culture.... actual names of real people) and why it was made besides the obvious of flushing manure out to sea.
100k years from now, the English language is still going to be understood by at least scholars and English Wikipedia articles about the Voyager spacecraft and possibly NASA/JPL manuals on its operation may even very likely survive to that time for future archeologists to be able to understand this artifact of humanity. If an alien species (aka something not from the Solar System) finds this spacecraft, they will have those scholars from the Earth to explain it and pull out those JPL manuals to even re-activate the scientific instruments after some repair of the vehicle and replacement of the RTG fuel. It will also be very well preserved in 100k years or even a million years.
Untapped mysteries of ancient cultures is just a scam made by people trying to sell books.... often about UFOs and other nonsense that is being generous to call pseudo science. While some thoughts about those ancient devices and buildings is conjecture and speculation, there is some sound reasoning as to why it is thought to be identified. I don't buy your premise here.