Originally (about 20 years ago) java threads were M:N I think. How is this different? If I had to guess, they are not opaque to the VM which has more freedom to optimize them.

This is mentioned in the article: the old green threads model was not blocking aware - if a green thread scheduled on an OS thread issued a blocking call (say a listen on a socket) than the whole OS thread was blocked.

With Project Loom, if a virtual thread executes a blocking call, the virtual thread is suspended and the OS thread is free to execute another virtual thread.

So how does it actually execute the system call under the hood? Using some kind of background OS thread pool?

From what I can tell, async system calls are used whenever possible. A blocking call on a socket doesn't make a blocking system call, thus permitting the carrier OS thread to go do something else. As for file I/O, things are a bit messy. Older linux kernels don't support "true" async file I/O, and the Java NIO async file channels do use thread pools to emulate async behavior in that case.

Green threads was only available on Solaris. They did not support more than 1 cpu. They were not particular particular performant either. And on top of that had issues with calling native code.

There was a before time when they weren't 1:1