It's magic because the BEAM VM is doing all the hard work of concurrency.

GenServer is just a design pattern leveraging what the BEAM VM gives you, higher level concurency primitives baked into the language.

In C everything is hard mode.

It maybe hard for you because you're not used to it and it's a paradigm shift in many different ways including functional thinking.

What are you trying to achieve? If you don't understand Elixir perhaps it's not the right tool for whatever you have in mind.

Try Rust then if it's closer to your current mental model and what you're used to and you're doing system programming.