Ubuntu's equivalent of the AUR, if I understand the AUR right, is PPAs, which are definitely not enabled by default and are fairly obvious about their third-party-ness.
(Main vs. restricted and universe vs. multiverse is just about licensing, not access control or vetting.)
There's multiple layers of problems here, because as we see with Ubuntu, just because the code was built and uploaded by a trustworthy person doesn't mean it's automatically safe or secure (especially for more nefarious infections that bury themselves deep in the source tree). Remember the pwnage that brought down kernel.org for a few months? That was only a few years ago, but the infiltration had been quite serious, and if I recall correctly, there was some concern that infected code had made it into officially distributed tarballs.
In practice, I don't know that the distinction between distributing packages that are trivially exploitable and distributing packages that have exploits pre-baked in is really that big of a difference. Automated scanners often pick up and automatically exploit exposed instances within a few days.
What it boils down to is that admins need to take responsibility for their own workload and what they choose to execute, no matter the guarantees of the distributor or the claims that $Sandbox_Y is magically impenetrable (which was silly enough before, but completely farcical in a post-Spectre world).
supported only community
free main universe
non-FOSS restricted multiverse