What TTS background does read-aloud use?

[BackMountainDevil]

At Readme

Read Aloud uses text-to-speech technology to convert webpage text to audio.

Read Aloud allows you to select from a variety of text-to-speech voices, including those provided natively by the browser, as well as by text-to-speech cloud service providers such as Google Wavenet, Amazon Polly, IBM Watson, and Microsoft.

I install speech-dispatcher(0.11.5-1), espeak-ng(1.51.1-2), Read Aloud-firefox(1.65.0) on my os. It seems that Read Aloud does not use espeak-ng. Because I test both on same Chinese sentences.

Can anyone tell me how it work? use some tts api provided by google, amazon?