Yesterday I downloaded the latest Manjaro/XFCE version in order to make a totally new rebuild of my system. I was very surprised to find that the Pacman contained in that version did not contain the access to the Arch repos.
In the prefs I could find Snap and Flatpac but NO Arch.
I finally went back to install an older version where Arch was included.
The work that I am doing REQUIRES the Arch extensions and I hope that this is not a signal of change in the Manjaro philosophy.
I think you’re referring to the AUR (Arch User Repository) which is indeed available via Add / Remove Software or pamac via terminal.
Did you install the base-devel group?
It’s a prerequisite for AUR
Just Enable AUR support in pamac preferences.
In preferences, select the AUR tab, and enable it from there.