What might be causing rear casette to creak? So I'm a bike noob in terms that my bike is certainly new, and since I bought it I've had problems with it.

So this time, I've figured where the problem comes from, it's the rear casette exactly, once it's under load, it makes a creak sound, which at high speed, sounds like a click, and in the past used to be just a click, very loud, it has been reduced to a softer creak now; let me make a timeline of important events.


It was clicking awfully and louder, when on load.
The bike shop changed the freewheel.
It started creaking softer, still would click at fast speeds.
The bike shop changed the pedals, bottom bracket.
Nothing happened, it was still the same if not worse.
I started playing with the rear cassete and derraulier, it got better for a little while; it got fixed the day it rained, somehow.
Creak/click is coming back, I found a way to replicate the load, and it replicates the sound, definitely the cassette; even grabbing the casette and rotating it with your bare hands makes the same sound; I was not strong enough with only one hand to do it in the video.


What do you think it is? should I complain for the 7th time in the bike shop but this time tell them exactly what it is? anyway the more information the better, so at least I can talk with property.

Here's the video where I show my homemade attempt.


                
            

Also I'm surprised this is exactly the same exact sound that it does when I climb (thanks youtube): 
                
             this is 100% the same.
It sounds like you have isolated the issue to the rear wheel, and probably the rear hub.

I doubt that that cassette or it's attachment to the freehub body is the cause of the problem. It may be the freewheel assembly, or the hub bearings, or even a crack in the hub body.

To rule out the cassette, you could replicate the creaking using a chainwhip tool as show in your second video, then remove the cassette and see if you can replicate it by torquing the freehub body. It sounds like you don't have the tools to do this though. Do you think you could you work with a repair technician at the bike store and have them use a chain whip tool demonstrate the creak with the wheel out of the bike?

You say the problem has existed since you bought the bike. Were all the previous unsuccessful attempts to fix the problem performed for free by the store you bought the bike from? (In order to make good a faulty product that they sold you) If so, you can try requesting that they replace the entire rear wheel, or at the very least the entire hub. You may or may not have success depending on how long ago you bought the bike and warranty it came with.