Preemptive threads Vs Non Preemptive threads

Question

No, your understanding isn’t entirely correct. Non-preemptive (aka cooperative) threads typically manually yield control to let other threads run before they finish (though it is up to that thread to call yield() (or whatever) to make that happen.
Preempting threading is simpler. Cooperative threads have less overhead.
Normally use preemptive. If you find your design has a lot of thread-switching overhead, cooperative threads would be a possible optimization. In many (most?) situations, this will be a fairly large investment with minimal payoff though.
Yes, by default you’d get preemptive threading, though if you look around for the CThreads package, it supports cooperative threading. Few enough people (now) want cooperative threads that I’m not sure it’s been updated within the last decade though…