A BEAM process is not an OS thread. The way I understand it, a BEAM process is just a very small memory space with its own heap/stack, and a message system for communication between BEAM processes.

The BEAM itself runs multiple OS threads (it can use all cores of the CPU if so desired), and the BEAM scheduler gives chunks of processing time to each BEAM process.

This gives you parallel processing out of the box, and because of the networking capabilities of the BEAM, also allows you to scale out over multiple machines in a way that's transparent to BEAM processes.