Sharing my experience here on m1 mac.
Installing .net 6 basically deleted my existing .net 5 install on MacOS.
After install .net 6 rtm for both x64 and arm, I had to try to install 5.0.402 multiple times otherwise trying only did nothing, then .net got corrupted and i had to ran the .net 6 arm installer again.
Now everything seem to work ok, but to install the x tool which require .net 5, you need to run the install command as follow.
dotnet tool install -g -a x64 x
If you dont specify the arch it will try to look for a .net 5 arm runtime which doesn’t exist.