yay builds AUR packages locally from PKGBUILDs. Use pacman for official Arch packages and yay only when the package is not available there. AUR packages are community maintained.
Install
sudo pacman -S --needed git base-devel
git clone https://aur.archlinux.org/yay.git
cd yay
makepkg -siUsage
yay -S <package-name> # install from AUR
yay -S google-chrome # example