Manjaro is “Arch derived” but is not Arch. Manjaro maintains its own package repos. And one of the big differences is that the packages in Manjaro are held back a few weeks before release. This difference in base repositories can matter if you try to use the AUR.
In many ways, EOS is not even a distro. It uses the Arch repos unmodified. It uses the Arch kernel unmodified. You could say that EOS is an opinionated Arch installer with pragmatic defaults. EOS has its own repos but there are only handful of packages in them, most of which are optional utilities or theming. Once installed, EOS is essentially Arch. As such, it is 100% compatible with the AUR. Two of the packages in the EOS repos are yay and paru which means the AUR works out of the box (unlike Arch itself).
You may think I am being unfair to EOS. It is my favourite distro. Manjaro is the only distro I warn people not to use.