Is there an nextcloud app like this one for manjaro arm? OpenStore
I’m not aware about a specific nextcloud app. What exactly do you need from it. Calendar and Contacts syncing with nextcloud works out of the box. Just add it in Settings → Online-Accounts (I guess it’s called in english). For Files you can directly mount it into your Filesystem and access it like an regular Folder. Afaik sadly the default File-Browser portfolio can’t do it via GUI but many other like GNOME Files or caja etc. can do it.
For example the nextcloud app for android allows me to choose what to sync and which folders…setting the nextcloud in online accounts on gnome settings is more limited.
if mounting the folders isn’t acceptable for you, you could try the official nextcloud client for linux.
sudo pacman -S nextcloud-client