I found that on Archlinux, AUR helpers like pamac and yay are not packaged in official repos
I am wondering if we can install pamac like this:
pacman -S --needed git base-devel
git clone https://aur.archlinux.org/packages/libpamac-aur.git
cd libpamac-aur
makepkg -si
pacman -S --needed git base-devel
git clone https://aur.archlinux.org/packages/pamac-aur.git
cd pamac-aur
makepkg -si
No, they wrote how-to just for joke. You have to install it via some undocumented way.
By now you should understand what AUR is and how AUR works.
Besides that, why are you asking question about Archlinux on the Manjaro forum? Or is the question how to install Pamac from the AUR in Manjaro?
pamac is already pre-installed in every Manjaro OS.
Nobody asks about how to install pamac in Manjaro forum except other distro users.
Either way it is non sense, if this is for Manjaro despite the title (the thread itself doesn’t clarify anything, just by title we can assume it is for Archlinux, but often we can see thread title and thread intent are different, hence the question about clarification), or if you have a question about Archlinux (this should be asked on their forum, and you will probably get ass kicked for asking such question).
I did not say about it 
I mean, who want to ask “how to install pamac in Manjaro forum” if he is Manjaro user.
Ask Romans how they do at Rome.