Using voice input in ChatGPT works on Android as follows:
Tap the text field to bring up the on-screen keyboard.
Look for the microphone icon on the keyboard. It's usually located in the top row of the keyboard or next to the space bar.
Tap the microphone icon. This activates voice input.
Start speaking. Your words will be automatically converted into text and inserted into the text field.
When you are finished, tap the microphone icon again to end voice input.
Ensure your device is connected to the internet, as voice input typically requires an online connection. Also, check if the voice input settings on your device are correctly configured, especially if you wish to speak in a language other than English.
Admittedly, an improvement would be a microphone icon in the input line of ChatGPT, similar to WhatsApp.
A similar topic is discussed here: