Is it possible to create a dropdown list from only one cell as a source? Each element separated by commas?
This isn't a built-in Google Sheets web app feature but by using Google Apps Script it's possible. To learn the basics, please checkout https://developers.google.com/apps-script/guides/sheets
Not possible via Data Validation. You can either use List of items mod where you enter your comma separated values directly, or you can use List from a range mod where each dropdown item needs to be in a separate cell.
A workaround would be to have your one cell with CSV items as you wish to, and then secretly deploy that one cell into a range and feed your Data Validation from that range referencing your one cell. Something like this:
C1: comma separated values in one cell