Lightweight processes is explicitly not os threads in at least two senses: smaller footprint in memory, and no system call for every context switch.
It’s explained in many documents about lightweight processes, of course for elixir/erlang/beam but also for Go and Crystal and even going back to Solaris Internals and modern Project Loom for upcoming JVM situations
Erlang's beam processes are undoubtedly awesome. But if we start with the premise of "achieving the same goals without Erlang" I think it's entirely valid to start with "what does our process primitive look like". With just 16GB of RAM my laptop is quite memory constrained, but according to task manager I'm still running 5200 threads accross 350 processes right now. Many use cases that required light threads/processes 37 years ago or even 20 years ago would work with OS threads by now. Of course many others don't, which is where the Erlang popularity comes from.
It takes on the order of nanoseconds to start an Erlang process. They are also extremely lightweight memory-wise. And Erlang VM tries to keep context switching between CPU cores to a minumum. And all processes get more-or-less equal share, so it's hard to get a process consuming all of CPU and never yielding back to other processes.
So firing off and monitoring processes becomes second nature easily. Rarely so in other languages
