Are you able to get apps from openrepos.
When downloaded from openrepos you should see downloads from jolla browser menu and install.
devel-su , needed only for special things, only, see that basic operations are ok first.
You do know there is a screenshot function and remote (ssh) access to the phone?
While the file browser is nice and all, it is third-party. There is very likely a warning about that the store version has limited capabilities.
When you are getting started, i strongly recommend using the phone as intended - i.e. find your downloads in the download manager, either from the browser or from settings.
pkcon install-local takes a file name - it looks a lot like you are not giving the complete file name (i.e. missing the .rpm ending). Never type out full filenames (or paths) - always use tab to auto-complete. And if it isn’t autocompleting, it isn’t there.