How did I get a flatpack installed?

I usually use the net installer, but it's possible that I used the Workstation live to install F34 on this system. I noticed Thunderbird seemed to be a bit strange, not integrated well. For example, I couldn't open pdf files normally. I couldn't pick the pdf viewer, only an option to use the default system opener or save the file. I discovered what had happened when I tried to update it and dnf said it wasn't installed. It took a while, but eventually I figured out that it was a flatpack. I would never intentionally install a flatpack, particularly when there's a perfectly good rpm available. So my question is, how did Thunderbird get installed as a flatpack? Is that the default for the workstation install now? When I opened Gnome Software, it gave me the initial welcome dialog, so I didn't use that to install it. I did use that to remove the flatpack and I could see that it would be very easy to accidentally install the flatpack version instead of the rpm one.