terminal cannot navigate shares on Big Sur mount a share like this:
sudo open smb://user:password@NAS/share

navigate to share
cd share

try to list contents
ls
ls: .: Operation not permitted

EDIT: clarification thanks to Greg's comment. 'sudo ls' fails as well.
In either case - using open smb://user:password@NAS/share or sudo open smb://user:password@NAS/share the folder is browseable in the finder but not in terminal.
It doesn't matter what user run the open command, it was mounted for everyone.  The sudo is a red herring.
To navigate to the share use cd /Volumes/share.  You can check the mount point by running the command mount.
Now the contents are visible running the ls command, subject to the permissions of the user running ls.