It would be interesting to see data on what percentage of fast food workers are part-time vs full-time. I tried to find some data, but came up empty. I did find this article from 2018 that gives some insight on fast food workers:
Things You Don't Know About Fast Food Employees
It does say 87% of fast food employees receive no benefits and over $7-billion per year in gov't welfare goes to fast food workers (2018 numbers). So, for a lot of these franchises, they are pretty clearly propped up by government assistance.