useCallback

Yes, it can be used to memoize both synchronous and asynchronous functions
Yes, useCallback can be used to memoize both synchronous and asynchronous functions. It memoizes the function reference and updates it only when one of the dependencies changes. However, it does not memoize the results of the function itself, so additional techniques like caching might be needed for result memoization.
No, useCallback is only for synchronous functions
It depends on the specific use case
Asynchronous functions are automatically memoized in React