This is a client-side page that uses the OpenAI Whisper API to generate text from speech.
You need to insert your own API key for it to function (and so you pay for whatever you use directly to OpenAI). Go to OpenAI API Keys page to get your key.
The audio data is uploaded directly to OpenAI. They impose a limit of 25MB as of the time of writing this.
For pricing, see OpenAI Pricing (scroll down or search for Whisper). For more API details, see API Documentation.