If you are using ACF (Advanced Custom Fields) and want to output the image alt text in the custom alt text field, it’s quite simple, but I could not find anything when searching online. So here it is.
If you have an image in ACF to refer to, it would be something like this:
{acf_image}
and to get the image alt text would be:
{acf_image:image_alt}
This is based on the dynamic data filters in the docs: Dynamic Data
Hope this helps.