Confused after migrating files from Mojave to Catalina

I just set up a volume with Catalina on my MacBook Pro running Mojave. I then ran Migration Assistant to move over all but the applications. While doing that I had to create a user which I named mojavetransfer. I would like to have everything in the main user, since I'm the only user. Can I just move all the documents I need manually over to the main user and then remove the new user? Also, it seems like when I make a change in Catalina, it makes the change on my Mojave volume as well. I just don't want to mess up my Mojave volume since that's where I intend keep doing most of my work. I only got Catalina so I can run TurboTax 2021.