phone keyboard as generic usb keyboard?

[VaZso]

is the phone keyboard a full windows keyboard?

If so the keyboard has an usb header, if costs would be within reason,
the phone keyboard could be redesigned into a multi purpose keyboard.

What you mean about full Windows keyboard?
People usually refer full Windows keyboard as a regular keyboard which has a special button with a window as illustration.

However, standard keyboard used a serial connection and newer keyboards use USB connection which also has standard behaviour and sometimes a more advanced one.

Pinephone's keyboard is absolutely not a standard keyboard.

Simply it has a key matrix and a microcontroller which handles its rows and columns.
It's main purpose is to identify pressed keys (using rows / columns arrangement) then communicate it as an I2C slave device.

So if we are speaking about an USB connection, it depends on the microcontroller itself, the hardware design and also the firmware running on the microcontroller if it is capable to present itself as a USB HID device.

I haven't read too much about the microcontroller used in this device but as far as I read, it is absolutely not a common part (practically it was also designed in China) and although it supports USB communication it may be not easy to do it using an uncommon microcontroller.

Usually simplest microcontrollers only have USB device support and more expensive microcontrollers have both device and host support.
Device can be connected to a computer (like keyboard, mouse) and host may have a device connected (like bluetooth receiver, keyboard, mouse)...

So, PC usually only acts as a host and basic peripherals are devices.