There are many reasons to dislike Nvidia on Linux. Here is a little thing that bugs me all the time, the updates. Normally the system updates would be quick and fast, but with the proprietary drivers of Nvidia involved, it gets quiet slow process. And I am not even talking about any other problem I encounter, just about the updates.
As an Archlinux based system user (EndeavourOS to be precise), I get new Kernel updates all the time. That means every time a new Kernel version is installed, the Nvidia driver DKMS has to be installed too. And that is basically the slowest part. But that's not too bad, even though it's doing this twice for each Kernel I have once.
What's more infuriating is, if you also happen to use Flatpaks for a very few applications. I really don't have many Flatpaks at all. Yet, the Nvidia drivers are installed in 7 versions or what?! And they are full downloads, each 340 MB or more. This takes ages and is the only part that takes long to update Flatpak system. I always do flatpak remove --unused to make sure nothing useless is present. /RANT (EDIT: Just typos corrected.)
Yeah no, most build scripts if they are worth their salt, will absolutely not pull the latest package from a given source. Because that is insane, 99% of the time they validate the download with a checksum, meaning that you have to update the checksum in the build script, or in the case of multiple downloads - multiple checksums.
Yes pacman is the underlying technology that enables aur to exist.
Those ideal build scripts would also get updated when the the source published a new version, but alas, being maintained by random users means the build scripts not updated in timely matter sometimes. In that situation, having another build scripts that say, pull git head, is often useful than none at all. It's up to the users to evaluate and use whichever is appropriate for them.
You are just guessing. No, they manually update those, because you need the checksum.
A tip never use any tool that downloads something without checking a checksum, because you have no idea if the source you are downloading is still the same, it could be anything.