I got the UB Ports Community Edition of the PinePhone and found Ubuntu Touch to be a very unpleasant experience and not at all what I signed up for. Luckily postmarketOS was exactly what I wanted and I moved to that within a week or so of owning the device.
Ubuntu Touch mounts stuff read-only, pushes you into using webapps, discourages using normal shell stuff like you would on a desktop GNU/Linux system, and has some poorly-working container system for if you wanna use shell stuff. It honestly felt worse than using Termux on an Android phone.
With pmOS you can install nvim, firefox, mpv, minetest... Things just work how you'd expect. It's like a single board computer (raspberry pi, etc.) that comes with a screen and other things already attached. You can ssh into it and update your packages all at once. It's really nice.
I think it's a stretch to say they're starting from scratch, but if you just mean the UI, Manjaro supports Lomiri (the UI from Ubuntu Touch) while also giving you a more normal OS experience. I'm glad to have some diversity in the desktop environment. Phosh is one of the more polished options, but has the issue of being GNOME-flavored and having all the issues you'd see with regular GNOME. Plasma Mobile feels more Android-like and familiar, but is still rough around the edges. Eventually it will hopefully be as good as Phosh and people can choose whatever they like the look and feel of more.
I suspect Pine ships Manjaro ARM because that distro will apply nearly any patch to the kernel if it makes something work, no matter how hacky or unsustainable the patch is. Right now their kernel has 43 patches, and they are not even organized or annotated well.
They ship Manjaro ARM because the people behind PINE64 are manjaro fans and they think that Manjaro has a good business setup. It would have been much nicer to have PostmarketOS shipped stock.
Pine64 itself is a hardware company. The software side is community-driven, and UBports (the old Ubuntu phone OS) is one of the operating systems you can install on your PinePhone.
Why are projects like this starting from scratch and not forking from the abandoned Ubuntu phone OS?