Yeah its very hard to understand as there are many writing errors, without any offense as this is completely normal.
builds adapted ROM
GrapheneOS is not a ROM. ROM (read only memory) is just a small part of the firmware, that, as the name suggests, cannot be changed. "Custom ROM" makes no sense.
Device manufacturers can generate GrapheneOS ROMs by simply providing their hardware configuration on the official GrapheneOS website, (CPU, GPU, Display, Battery, flashlight, Camera. boot-up animation, etc.)
The problem is not (only) that most of these infos are proprietary, but also that no device matches the security standards of a Pixel with full support for custom OSses. Samsung nearly matches them, but hinders 3rd party OSses from using these features.
GrapheneOS officially pushes out updates, device manufacturers do not have the right to do so
Why would any manufacturer want that?
the device needs to be tested by the Customize OS
Very hard to understand what this means, but the Customized OS would need to run and support the hardware, yes
plugging in the "OTG USB 128G" OS
Are you referring to an USB storage with 128GB size, connected over OTG? An OS will not be stored here.
people can freely choose any OS distribution version
So they choose the wrong one and the device is soft-bricked and doesnt boot? I dont know what versions you mean, as its just GrapheneOS+drivers as far as you detailed it. No need to choose if only one version works
users can set the Bootloader UnLocking password.
I am not sure how this is done, but normally you connect to the server of the carrier (as far as I know) and they allow you to unlock the Bootloader. You need the unlock authentification, nothing custom.
Not sure how unlocking is done with GrapheneOS installed, probably works without that connection, but a "custom password" does not exist.
So yeah many things dont make sense. You dont need Distributions if only one works. You dont have install media on USB flashdrives like on Desktop Linux.