RTEMS isn't "controlling" the execution of the code, it just supplies implementation for OS-specific utilities (effectively the POSIX API). The BEAM VM code is compiled to machine code and runs directly on the CPU. As peerst explained, if you write your own C code in an Erlang port driver for example, you don't have to use RTEMS at all.
You can only run one process in RTEMS (hence the name The Real-Time Executive for Multiprocessor Systems), however you can simulate other processes via threads. This is how we run the network stack and Wi-Fi configuration tools among other things.
