Not knowing that much about apt, isn't _any_ package system vulnerable, and purely a question of what guards are in place and what rights are software given upon install?

It's not the packaging tech. Apt will typically mean a Debian-based distro. That means the packages are chosen by the maintainers and updated only during specific time periods and tested before release. Even if the underlying software gets owned and replaced, the distro package is very unlikely to be affected. (Unless someone spent months building trust, like xz)

But the basic takeover... no, it usually won't affect any Debian style distro package, due to the release process.