PythonSDK async mechanism of TTS & STT : is async achieved by multi-threading or multi-processing?

[ANYMS-A]

I'd like to know that for python sdk's async TTS or async STT, is the callback fucntion task be executed by a sub-thread or a sub-process? (e.g. .speak_async( ), .start_continuous_recognition_async() methods of SpeechSynthesizer and SpeechRecognizer class)
Usually, I will try to collect the output of the task in the callback function like

recognizer.recognizing.connect(collect_data_callback_function)
recognizer.recognized.connect(collect_data_callback_function)

Are those callback functions executed in the sub-thread?

I ask this is because the GIL limitation in python: if I execute some thread-blocking operation in my main-thread, will it block the execution of the callback function? If it will, I might need to consider the move those operation to a sub-process for execution

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