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Not necessarily. Concurrency is generally not parallelism.

Coroutines help solve IO-bound issues - a long networking API call doesn't block your program. You can continue doing other things. The side-effect of which may use all your cores efficiently but that's not the primary goal.

No. Think of the coroutines as goto:s with a dynamic label that does some magic to replace locals too.

You still need threads.

it'd be very trivial with something such as ASIO's io_context as your coroutine event loop: https://dens.website/tutorials/cpp-asio/multithreading