your link does not actually work for me (gets a 404) i guess you linked the “thank you” page that comes after the download started …
anyway, if you are sure this is Ubuntu Core (which normally should say “Welcome to Ubuntu Core 20” after login), you will be able to use sudo with the user you created without specifying a password…
though there is no reason to use sudo normally at all, your user is logged in to the snap store so all snap commands should work without elevating permissions and you should not actually manually modify any files since they might be managed through a snap or snapd … that said, something like “sudo vi /etc/ssh/sshd_config” should surely work …
also note:
$ snap info nano-strict
[...]
This snap ships GNU nano in strict confinement mode, this means that it can only access
non-directly-under-hidden-files under your home directory and (optionally) files under `/mnt` and
`/media` via the `removable-media` interface
[...]
$
