Why did I end up with duplicated apps when I upgraded macOS from Catalina to Big Sur?

When I updated my iMac (27in,2020) from Catalina (10.15.7) to Big Sur (11.7.10), I ended up with (mostly) duplicated apps in Applications folders under both the System directory and the Users -> username directory!

Can I just delete the duplicates, presumably from the username/Applications folder?

Plus...

To be safe, I downloaded a full installer for Catalina and created a bootable flash installer drive for it, but now I can't delete that installer app because it's "in use" (see attached)!

Since I'd done the same thing with Monterey, Ventura, and Sonoma (for future upgrades) and had no issues deleting those installer apps once I created bootable drives for each of them, I don't understand what was different (other than there seemed to be some confusion over the full installer file size, see attached excerpt).

Thanks in advance for any helpful advice!