So sorry where are /var/www/ and also nextcloud files stored?
/var/www is stored on your root partition, so the storage medium you installed/flashed DietPi initially to. Based on your lsblk output this should be the SD card indeed.
The Nextcloud data (not the PHP files and website resources, but your userdata) are stored at /mnt/dietpi_userdata/nextcloud_data. That way you can move it together with other userdata to any external drive via dietpi-drive_manager, selecting the desired target drive and then the “Select to transfer DietPi user data to this drive” option. After done, /mnt/dietpi_userdata/nextcloud_data is still the valid path, but you will see that /mnt/dietpi_userdata has become a symbolic link to your chosen external drive:
ls -l /mnt/dietpi_userdata