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.
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.