That I can help with.
This script is being called:
https://raw.githubusercontent.com/tteck/Proxmox/main/ct/nextcloudpi-v4.sh
Look close to its bottom, the main install command is
lxc-attach -n $CTID -- bash -c "$(wget -qLO - https://raw.githubusercontent.com/tteck/Proxmox/main/setup/$var_install.sh)" || exit
For $var_install look at the top, with the given variable this is “nextcloudpi-install”.
And with that you can complete the path, this is the install script:
https://raw.githubusercontent.com/tteck/Proxmox/main/setup/nextcloudpi-install.sh
And in this you see where it is loading NCP from:
msg_info "Installing NextCloudPi (Patience)"
curl -sSL https://raw.githubusercontent.com/nextcloud/nextcloudpi/master/install.sh | bash &>/dev/null
msg_ok "Installed NextCloudPi"
Looks official to me 