I can download Survey123 data into csv files.
I can also download photo files via the REST api.
I cannot however download the signature captured during a survey.
I can see the signature when I select the individual data rows on the Survey123 site.
Is it possible to access the signature file(s) via the api so that I can download them (without having to use the ArcGIS desktop application)?
You should be able to get the signatures via the REST API as well (they are stored as images). See https://community.esri.com/groups/survey123/blog/2017/03/22/working-with-your-survey123-photos for more information.