As far as I know BigWorld was open source several years ago, but now it is closed. So it is not used by other companies. It is written in C++, game logic could be written in C++ or Python.
Erlang is not used for game backend server. We have serveral databases: PostgreSQL, MySQL, ClickHouse and others.
Machmaking is probably part of BigWorld, I don’t know for sure.
For web apis we use Erlang/Cowboy or Elixir/Plug (no Phoenix). JSON is the main serialization format, also we use other formats also, like message pack and avro.