> In addition to that, the source code is closed and not generally auditable by third parties.

That's not correct. While you can't get it from GitHub, there is a process to audit it and any Brazilian citizen (or resident, I'm not sure) over 21 can request and be part of it. The process extends for many months starting the year prior to the election. Input from the multiple audits and tests are valuable in guiding the evolution of the software and hardware.

In Portuguese: https://www.tse.jus.br/internet/temporarios/urna-seguranca/o...