nheko (https://github.com/Nheko-Reborn/nheko) is in extra (https://gitlab.archlinux.org/archlinux/packaging/packages/nheko) while nomacs (https://github.com/nomacs/nomacs) is in the AUR (https://aur.archlinux.org/packages/nomacs).
If it's in the AUR, get it from the AUR :)
As for nheko, it has a few dependencies that aren't already in our repos. I started looking at it and brought in a package, but I can't make it a priority with so many packages without a maintainer. I may get to it, I may not. No promises.
In the mean time, OP could just enable the Arch repos (https://wiki.artixlinux.org/Main/Repositories#Arch_repositories).
It would be nice to see nheko in ARMtix repo also.
For some reason Arch Arm doesn't have it.
But it works fine on arm fine other distros have it and you can build it.
My PinePhone after 3-4 hours, with GUI closed and services stopped, didn't finish to compile it.