I honestly don’t know why this gets asked about as often as it does, but I’ve seen several instances in which Mac admins want to know how to get the Rosetta 2 installer .pkg to be installed offline. The installer is about 6.2 MB, so I don’t think bandwidth is the issue, but maybe it is.
In any case, if you have a Silicon-chip Mac with an active Internet connection, run
softwareupdate --install-rosetta --agree-to-license to install Rosetta 2. Then, run
grep "RosettaUpdateAuto.pkg" /var/log/install.log to find where the installer downloaded to.
The location will likely be something like
/var/folders/f5/_hdu19hcuin1ckjnqkjcndwkcnadskjnckjqwn/T/OAHSoftwareUpdate/RosettaUpdateAuto.pkg. Go to wherever it is in Finder, and then copy that installer .pkg to wherever you want it to be.
Note: I’m just telling you how to locate the installer. I’m not vouching for it working offline, since I haven’t tested that.