HW / FW build and toolchain setup

[nbgsmk]

Hello my friend!
I will try to build the MMDVM board with your gerber files and compile the firmware from source. There is a lot to learn in the process, and it's a nice challenge for (admittedly advanced) amateurs.
But ofcourse I ran into some difficulties (and anticipate others). So could you please help with a few things:
HW

• are your gerber files good for producing a pcb without any changes? I am thinking of a minimal quantity with JLCPCB and their smt assembly process.

FW

• what is the reason for choosing stm32f105 instead of 32f103c8 (like practically all others I can see online)
• what is your development environment: windows or linux?
• in file->preferences, what is the address in "additional board manager urls" ?
  (After installing the board support according to some instructions on the net, I was able to build arduino due. But after installing the lib and switching to stm32f103, 105, 407, 411, they all fail with "unknown processor type". I followed g4klx's build instructions (many times:) ) but without success. Tried older versions of arduino ide, modified platform.txt etc..but the build still fails.)
  In the latest attempt, the f105 cpu is not even available in the "board part number". I have only F103 variants, so I must be doing something wrong.
  So, what goes here?
  

And what goes here?

• finally, do you compile from arduino ide -> sketch -> verify/compile or from the command line (and how exactly)?
• if you also tell me what is the modification in your platform.txt, I promise a big cold beer will be waiting for you in my fridge. :) :)

I am using the latest arduino ide 1.8.13 via installer.

Thanks!