So I have installed the docker version on Ubuntu and the files are not in any of the suggested directories.
plxadm@plex:~$ sudo ls /var/snap
core18 core20 lxd snapd
plxadm@plex:~$ sudo ls /mnt
plxadm@plex:~$
Has anyone got any better ideas?
hello @tune welcome to the forum 
I split you post into separate topic as it not related to snap installation as previous thread.
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you find information about official docker image and volumes it uses are here GitHub - nextcloud/docker: ⛴ Docker image of Nextcloud
Please use the search - lot of issues have been discussed already.