where did you get this understanding? this is definitely, 100% wrong, C++ coroutines are not backed by threads at all (or by anything really since as of C++20, the language only provides the building blocks for making your own coroutine / generator types and not actual implementations).

I haven't read much about them, but it looks like they can be driven by any event-loopy thing. It's not like creating a coroutine creates a thread.

https://en.cppreference.com/w/cpp/language/coroutines