iSH does allow you to download packages from the Alpine package repo, but they maintain their own mirror. The only issue is that they haven't updated it in a while, so its stuck at Alpine 3.14, and there's no (at least straightforward) option to upgrade to the later versions or to Alpine edge. I haven't yet tried updating the /etc/apk/ files to make Alpine upgrade though.
This isn't real "arbitrary package downloading", though — Apple still audits all the code that goes into these repos (which is why they can't just keep them up-to-date.) It's essentially just offloading of some of the app's packaged code into separate "DLC" modules, to make the base app download more lightweight.
> I haven't yet tried updating the /etc/apk/ files to make Alpine upgrade though.
I would highly suspect that this wouldn't work.
Maybe it would have in some prior version, back when there was the technical barrier of it being very hard to cross-compile Linux binaries for arm64.
But I would guess, upon the popularization of things like the Raspberry Pi, that Apple required the developer of iSH to modify the version of apk(1) that ships with the tool to only work with APKs that have been signed by Apple.
