Rita, after all your download problems I went looking for an alternative and now with the help of
Diskmaker-X-9-Catalina, a donation-ware Applescript utility, and
this YouTube video on using it, I have a 16GB thumb drive with the Catalina installer on it. Not that you still need it, but there may be others reading this thread who might be interested.
Now I will have to see if it will work for Big Sur.