Ancient giant Elbaph is likely from the second world. One key point from the Harley text 'Sun lit the ambers of war' which is likely means the God of War Elbaph and Joyboy destroying the corrupted kingdom the oppressor and liberating those who were oppressed. They mostly active in the New World like Imu said they used to dominating the New World.
Uranus and the Sun should already be in possession by someone. The person used power of the Sun to charge the Ancient weapon Uranus to destroy the world. While the sea level rises, many people seek shelters on higher ground, Adam Tree is one of them. The giants race also scramble to find shelters. They fought everyone under the Adam Tree and came out victorious. Elbaph named the place after himself.
Weird thing is that the harley text's dragon has it's horn on different direction and we don't see it from Loki.
The giant race seems to be savage but some were actually smart, they discovered the giant robot and created a lab from it's left over scraps. I believe they are the Galleila group. They learnt everything from the ancient robot. The world probably have already sunk half way. But they travelled to water 7 to build Pluton. So Pluton shouldn't have anything to do with the world sinking and wasn't used for destruction. It probably was used to terraform land. Imu or Davy Jones fought them and they lost. Joyboy recovered Pluton somehow.
From the mural, the dragon was fighting the flame serpent. I think they are probably not devil fruit yet, but could be a dream manifested into real world through very advance science, power of love, buccaneer's soul, and by using the energy harvested from the earth's core. These elements are what Vegapunk hasn't discovered yet. Yet from his limited knowledge, he manage to create the punk hazard dragon which looked quite similar to the one on the harley text, possibly an inspiration.
The Rotatoskr was probably created the same way, but lacking the formula of fruit in the mix, that's why it doesn't reincarnate into a fruit I supposed. Maybe some first world technology was lost when it was destroyed. What's intriguing is his soul reside into his master's favorite possession, similar to how Kuma's soul works. Kuma is actually dead and his soul left his body, but later came back and reside into his own body. We don't see Kuma gets a transformation because that's his body. This might be the true power of the power of love.