There are plenty of guides if you search for RosettaUpdateAuto.pkg. e.g.
https://tinyapps.org/blog/202103270700_backup_rosetta2.html
Download and install Rosetta 2:
% softwareupdate --install-rosetta --agree-to-license
Find path to installer:
% grep RosettaUpdateAuto.pkg /var/log/install.log
Copy installer for safekeeping:
% cp /path/to/RosettaUpdateAuto.pkg /path/to/backup/dir/