Avoid the AUR and use this instead. It is actually supported by the real MultiMC maintainers and community.
You need to set up your Arch install to be able to build packages.
See instructions for that: https://wiki.archlinux.org/title/Makepkg#Usage
If you have git installed, you can just clone the whole repository.
git clone https://github.com/MultiMC/multimc-pkgbuild.git
cd multimc-pkgbuild
makepkg -si
If you do not have git installed, you can use a downloader like wget or directly download the PKGBUILD using your browser.
mkdir multimc-pkgbuild
cd multimc-pkgbuild
wget https://raw.githubusercontent.com/MultiMC/multimc-pkgbuild/master/PKGBUILD
makepkg -si
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