where and how to use autotune in audacity to improve the quality.
Autotune does not support Audacity.
For a list of apps supported by Autotune, see here on the Autotune website: https://www.antarestech.com/host-daw-compatibility/
Other pitch-correction plug-ins are available,
some are free, e.g. … https://www.auburnsounds.com/products/Graillon.html
There are some free alternatives available that sound quite similar.
Just search for “free auto tune plugins” or something like this on google.