Aloha,
why are there different versions in extra-v3 than in extra?
Is there really a (significant) difference between for instance 0ad version a26.18 in extra and version a26.18.1 in cachyos-extra-v3, or is it just the naming? Should i always use cachyos-extra-v3 versions, when available?
I’m still new to Linux / Cachy and find this a bit confusing.
a26.18.1 is newer. The minor .1 version increase shows bugs or something else was fixed.
Yes, -v3 is compiled with x86-64-v3 cpu optimizations → more tuning to your cpu performance advantages. It is preferable to the normal versions.
Unless the v3 version is outdated compared to the other one (which shouldn’t normally be the case… rarely it fails to build, etc…), it’s better.
This is an overgeneralization and in the context of CachyOS is incorrect. -X increments indicates the Xth build that is pushed in the repo. Rebuilds can occur during a major dependency update (think Python 3.13) or as mentioned, something was cherrypicked or reverted to fix a bug that is present in the current version. For cachyos-{core,extra}-* repos, the package versions are incremented with an additional point number instead of bumping the release version, because nothing was changed from the Arch PKGBUILD, except for the fact that the package was built with higher CPU optimizations.
Nope, the answer was the right one according to what was posted. 
But yes you are right, as you can see in my post, I though it was a26.18.1 release version and not a26-18.1. → a mistake the original poster made.