For the AUR packages, you need to git clone them and compile. See the whole process here:
https://wiki.archlinux.org/title/Arch_User_Repository#Installing_and_upgrading_packages
Or more simply at the beginning you can use an AUR helper. There are many. E.g. paru.
paru audacium
paru audio-recorderer
...
You'll be presented with the PKGBUILD.
Scroll/check It and quit with q to proceed.
If you want to use octopi, make sure that an AUR helper is enabled in the settings.