It could have been possible if we were originally visitors from another world and using the latest technology that without the infrastructure in place would have perished over the years (how long would our vehicles, electronics and technology last if it was no longer possible to source and manufacture materials after a complete collapse of the infrastructure?)

I'm not talking about a few passengers and crew, I'm talking hundreds of thousands of people who had been travelling through space for thousands of years and several generations, some of which would no longer remember where they came from, on a spaceship the size of the moon. Vestiges of their technology, construction and drawings still remain......

These monuments, like so many throughout the world, shouldn't have been possible with the technology available at the time, remembering that these were built in the bronze age with no advanced hardened tools.

There really does have to be another part to the equation, for e.g. let's not forget such things as the granite boxes in the Serapeum at Saqquara, we couldn't do that today with all the technology to hand... !

Barry, the technology required to produce those granite boxes is not at all difficult. Physically difficult ? Absolutely , yes. But not technologically difficult.
Example: if given thousands of laborers, I could easily create a perfectly flat stone foundation, (out of bedrock) that measures 10 hectares square. You may ask how ? Simple. I'd start by digging a shallow square trench in the bedrock and fill it with water. As you know, water seeks its own level. It is then only a matter of chipping away and polishing the bedrock stone within the square. Eventually, I'd end up with a precisely flat and level bedrock, that varied only a 1/100th of an inch out.
The same applies to the granite boxes at Saqquara. simply invert the box lid and pour water into the cavity. Now, all that needs to be done is to slowly chip away and polish the bottom so that the water is at a precise depth everywhere. Reinvert the lid and place it on the coffin.
In 2007 I visited Saqquara Serapeum. Difficult to achieve those boxes ? Yes. But not impossible.

Oh, and let us not believe those who would say it's impossible with todays technology. That is a myth created by people who make money writing about aliens.

One of the greatest mysteries of human history is exactly how the Great Pyramid of Giza was built.
Herodotus, a Greek historian, writing in his seminal work, "The Histories", said it was built in aprox 2,460 BC., took 20 years to complete and contains some 2,300,000 stones. The average stone weighs 2.5 tons , but the single largest stone is estimated to weigh 60 tons.

2,300,000 stones in 20 years. That would be 115,000 stones cut and hauled into place every year. 9,583 stones/month. 319 stones cut and placed every single day of the month. Assuming the workmen labored 10 hours /day, that means they would have had to place 32 stones each and every hour.

32 stones every hour, rough hewn from the native rock, cut precisely , using copper hand saws, and hauled into place.

Perhaps even more mind boggling is exactly how the stones were risen to such great heights. Were stone ramps used ? Egyptologist engineers have concluded that such ramps would have taken material equal in volume to the Great Pyramid itself.

There are no ancient papyrus drawings or writings to explain how this was achieved.

The Great Pyramid of Giza, alleged tomb of Pharaoh Khufu AKA Cheops, the only remaining one of the "7 wonders of the world" stood as the tallest manmade structure for nearly 3,000 years.

As we see it today, is NOT how it looked upon completion. It was covered in polished white limestone and surely would have gleamed like a massive diamond.

As an aside, you might imagine that we would have numerous statues of such a mighty Pharaoh. you would be wrong. the only "statue" we have is a tiny figurine 6 inches tall. (see below)

Oh, dear. I was afraid of that. some people seem to think that if we have a great difficulty explaining ancient technology, then it must have been aliens.
Aliens ? Oh, please.

There's ancient wonders everywhere.Were they all built by Martians then ?
It was a popular belief at one time that the lines on the Nazca Plains were drawn by aliens but that has proven to be nonsense.

Oh, I think you can be guaranteed that it is solid. It would collapse otherwise.