I'm a software guy.
When I think about open source software, the first project that comes to mind is Firefox.
If I want my own instance of Firefox, it is easy to download everything I need from Mozilla and compile my own Firefox from source. Mozilla provides simple instructions and commands to copy and paste into my terminal to help me do so. Makefiles are provided to make this easy.
I can make changes to the source code and easily recompile if I want to make my own customizations.
This is how 'open' projects should work.
SatoshiLabs publishes their firmware source code and bootloader source code (finally). I can compile my own firmware and load it onto my Trezor, but I see an 'Unofficial' warning when running it.
I can compile my own bootloader, but I have no way to use it because the bootloader on a Trezor is locked and unmodifiable.
But the Trezor is also 'open source hardware', right? I can just build my own instance of the hardware that has not yet been locked and use my own compiled bootloader. Its 'open', so it should be easy!
The reality is not as straightforward. The only thing that SatoshiLabs provides to support their claim that the Trezor is 'open source hardware' is a pdf of the schematic:
https://doc.satoshilabs.com/trezor-tech/hardware.html
A pdf of the schematic is not enough to build an instance of a piece of hardware. I can't send a pdf of a schematic to a pdb fabricator and receive a working board in return. A pdf of a schematic is not a Makefile.
This would be like Mozilla saying that all we need to do to build our own Firefox instance is to write a program that implements the html and javascript specifications.
If Mozilla did that, there's no way anyone would believe their claim that Firefox is open.
This is what SatoshiLabs is doing with the Trezor. They are claiming that the Trezor is open source hardware and pointing to a picture of a schematic to substantiate their claim.
The Trezor is not open-source hardware. It is not easy to make your instance of a Trezor and incorporate your own changes.