...Yes, this is a new Mac. (how did you know?) I just got it two days ago. It came with Catalina...
I knew because your problem is fairly common here, so I recognized the symptoms which happen with those particular apps a lot. If you check at the bottom of the forum, I've been here from the start and am the top all time participant. I started volunteering in this forum when the Mac App Store was just public beta software.
Thankfully you are off to a good start with a new Mac by explaining your problem thoroughly right off the bat so that you and I didn't have to play 20 questions. It's common to just get, "I can't update apps! Why!!!!"
I have the red 1 flag for GarageBand on the App Store app as well. My Late 2012 Mac mini is too old to run Big Sur. However, My new Apple Silicon Mac mini will be here this Tues or Wed and it will come with Big Sur preinstalled. Big Sur seems to be the first step towards a merger of macOS and iOS, my favorite iPhone and iPad apps will run on the new Apple Silicone Macs.
I've been saving for this upgrade to Apple Silicon since Tim Cook announced they were coming back in June. I also have a new Philips 4K UHD display for my new kit arriving at the same time.
