It depends a lot on how the system was set up originally. Growing a virtual disk isn’t a big deal, but the steps are different depending on if LVM is used, etc. I normally use parted.
This is really not a question about Nextcloud at all since it will show the free space of whatever disk the data directory is on. You can follow any guide that fits your system, but remember to back it up before messing with it. Murphy gets bonus points for partitions.