I'd like to clarify a little what the OTP is vs the BEAM - this article does an OK job at that explanation but confuses it a little.
The BEAM is the underlying VM used by Erlang, Elixir and Gleam. The BEAM provides the basic primitives like spawning processes, sending messages, handling messages etc. Processes are lightweight pre-emptively scheduled tasks, similar to go-routines or green-threads in other languages. These primitives are mostly lower level than you want to deal with on a day to day basis.
The OTP is a standard library built on top of that, to provide a convenient, battle-tested way of building systems. The GenServer is the most well known component - it simplifies writing code that handles messages one by one, updating held state, and sending replies. And bits around it to the caller, sending that message and getting to the reply is just like making a function call. GenServers are then managed by Supervisors that know what to restart when something crashes.
One notable difference between Elixir and Gleam is that Elixir gets to just re-use the OTP code as-is (with some Elixir wrappers on top for convenience). Gleam concluded that the OTP is built expecting dynamic types, and that for best results in Gleam they'd need to re-implement the key primitives. That's why the example shown is an "Actor" not a GenServer - it serves the same purpose, and might even fit in a Supervision tree, but isn't actually a GenServer.
The BEAM is the underlying VM used by Erlang, Elixir and Gleam. The BEAM provides the basic primitives like spawning processes, sending messages, handling messages etc. Processes are lightweight pre-emptively scheduled tasks, similar to go-routines or green-threads in other languages. These primitives are mostly lower level than you want to deal with on a day to day basis.
The OTP is a standard library built on top of that, to provide a convenient, battle-tested way of building systems. The GenServer is the most well known component - it simplifies writing code that handles messages one by one, updating held state, and sending replies. And bits around it to the caller, sending that message and getting to the reply is just like making a function call. GenServers are then managed by Supervisors that know what to restart when something crashes.
One notable difference between Elixir and Gleam is that Elixir gets to just re-use the OTP code as-is (with some Elixir wrappers on top for convenience). Gleam concluded that the OTP is built expecting dynamic types, and that for best results in Gleam they'd need to re-implement the key primitives. That's why the example shown is an "Actor" not a GenServer - it serves the same purpose, and might even fit in a Supervision tree, but isn't actually a GenServer.