Debian’s official repos are so huge that they don’t need something like AUR. At the same time some long time Linux users tell me that despite the fact that Arch’s main repos are much smaller than Debian Arch has an advantage. The advantage that Arch has is better maintained repos. I was told that Debian has very very old and unmaintained packages in their repos.
Bottom line: I haven’t yet found a distro which excels in all areas.
The AUR also contains some proprietary software that one may need and packaged for arch, like RStudio or zoom. Debian repo is only free software. But there is the option to use flatpaks or app images, which can also be used in arch but doesn’t make proprietary software any safer…
The AUR is considered a big plus in arch based systems. But for beginners it is mostly advised to not use it blindly and make sure to follow some rules, like inspect the primary source from which the software was packaged etc.
As I said I can’t check the code. What I do before installing any package from AUR is ask myself “is this is a popular package ?” My thinking if the package is installed by many users at least some of them must have the necessary knowledge to review the code. I guess lots of Arch users must be using google-earth-pro.
I am not sure that the Arch repos are that much smaller to be honest. There are certainly packages in the Debian repos that aren’t in the Arch repos but there are also packages in the Arch repos that aren’t in the Debian repos.
Unfortunately, it is fairly difficult to compare repo sizes because the packaging is so different.
The vast majority of AUR packages don’t contain anything which requires special skills to check. The link above posted by @pebcak from @kresimir describes what you can check and how.
Some of the packages that are built from source are complicated and contain many patches and other things that can be hard to follow. However, these are exceptions. Most AUR packages are quite simple.
Checking most AUR packages doesn’t require a high degree of technical competency. It requires being willing to take the time read them.