So how do we explain giants, nephilim, rephaim and demons? Are the giants not the reason long with the corruption of man, for the flood in the Bible? Not with standing Enlil wanting to let man kind to be destroyed.

Giants and Nephilim were considered to be the same thing - the offspring of "fallen angels" or deserted Anunnaki/ETs mating with women that the gods aka the Anunnaki created. Some cultures call them and other creatures as demons. I'm pretty sure the Rephaim qualify as part of the Nephilim because they were the giant children of the Earth females+Anunnaki.

From what I've observed, for the religions and cultures that call any unexplained or otherworldly creature "demons", it's a very broad term. A "demon" can be anything from an ET encounter, a spirit (benevolent or malicious), an unexplained sound, an unexplained shadow, unexplained disappearance (like keys, or other items in the home that are not usually misplaced), etc.

The Giants were PART of the reason for the Flood. The average person was, as we're told, pretty sinful. While we don't know exactly what they did, scripture just tells us that they continually lived according to the whims of their heart, aka emotion (I think we can compare this when we see over-emotional protesters and how destructive the current narrative makes the general population). The Giants getting out of control, which I understand that to mean their population was growing in size and maybe they were forming their own armies, and most cultures claim that the giants ate normal humans, that was all PART of why the Flood was created "by God."

However, I think sin was the excuse for the flood happening. We humans like to rationalize everything to our personal scale, and the best way for a more primitive-mindset people is to say that "God" brought the weather (like they thought God CAUSED droughts and God CAUSED rain and God CAUSED earthquakes, etc). Considering the capabilities of the Anunnaki, I think it's safe to say that they were capable of SOME natural disasters, like flooding certain areas (which would cause a drought in other areas) and maybe things that FELT LIKE earthquakes. But I personally think the flood was caused by some giant meteor impact (I think Mr. Wallis said that might have been the Clovis comet?) because:

a) there was just enough time to build a watertight boat with enough genetic material to regrow the plants and animals afterward
b) at least one family escaped because they had been warned (but I think likely a few families, considering cultures all across the globe say their ancestors landed "here" after a great flood). For a technologically advanced civilization like the Anunnaki, they could have detected a comet that they knew they couldn't demolish before it hit Earth's atmosphere, so they would have had emergency measures put in place for their facilities (like labs) on the ground.
c) We also know the Great Flood only impacted PART of the globe, like the north-western side of Africa, much of north America, much of that land that is now little islands of Central America and the like. We know this because ancient Egyptians who were questioned about the flood were recorded to have said the flood didn't reach Egypt - of the land that is the Egypt we know today. An actual world-wide flood would have covered Egypt as well. It's certainly POSSIBLE that Egypt was flooded (and it would explain the water-weathering around the Sphinx) and the descendants of the people who re-populated the region of Khem and named it Egypt weren't taught about the flood so they thought the Flood never reached them (Which I think makes more sense than it not flooding, because the Sahara used to be a lush land with a huge jungle area and people used to keep livestock like goats - goats NEED grassy areas for grazing).

Anyway, those are some of my thoughts on all that. Does any of that help with explaining giants and the flood?