First update your post to list all of the components being used in the rebuild. Even though you point things out and mention names it was not (for me anyway) easy to follow.

Second, the video was revealing in other ways: you need to set up a better "workspace". I understand that sometimes you have to "make do" but overall all that TV tray work table is an accident waiting to happen.

In any work environment ending up with tools, components, wires, manuals, screws, etc. all scattered about just means that you will end up "fighting" with the process versus being able to "focus" on any immediate matter. Cables curl up, things start sliding about, stuff gets bumped, pushed, pulled - hard to control and eventually something will get broken.

What to do:

One question: any "beep codes" or "LED" error code lights on the motherboard? If so what are they?

Look the errors information up in the manuals: you may discover some simple error of omission or commission.

Failing that:

Take the system all apart very carefully. For now leave the CPU and cooling intact - that disassembly will be a last resort. Pull the CMOS battery to clear BIOS.

Then rebuild the system. This time just start with just the PSU, the motherboard, its' integrated components, the memory modules, and the boot drive. Be very wary about all of the power connections.

Inspect each part carefully for signs of damage, dirt or debris that may get in the way of a final fit. Do not overtighten screws.

Double check all connections, RAM seatings, jumpers. Check the applicable User Guide/Manuals as to the installation order and how each actual installation should be done.

Get the system to boot and function with the basic hardware and configuration. No OC, no RGB stuff. Basics only.

Once successful, then add in additional components; i.e., the GPU, audio card, Optical drive. One at a time.

And do allow some time between additions. At least a couple of reboots to ensure that each step does indeed work beyond just one time.

Get it all functional then move on to the customizations.