It is sad that Googling didn't find the answer by ChatGPT did.

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Here's an excerpt from the book "Java Threads" by Scott Oaks and Henry Wong, published by O'Reilly Media, which discusses green threads:

"Green threads provide a thread abstraction to the application, allowing the programmer to write multithreaded programs without worrying about the underlying OS support. However, because they're implemented at the user level, they are not subject to preemptive multitasking by the underlying OS. Instead, they must voluntarily yield control to other threads. If a green thread fails to do this, it can effectively freeze all other green threads in the system."

This excerpt supports the fact that green threads were not preemptive and relied on voluntary cooperation to switch control between threads.