Erlang is a much better language to learn if you're interested in learning about the BEAM and OTP, and the book "Programming Erlang"[0] is an excellent resource for learning it.
I've used Elixir since 2015 and in fact learned it first. I still think "Programming Erlang" is a much better book than any other for actually learning Erlang and BEAM/OTP principles. Erlang as a language is simpler, leaving more time and energy for learning the actual important bits about OTP.
I read Programming Erlang later, but it was just for fun, and I knew most things already at that point.