Googling this question gives a wide variety of results but I don't see anyone actually asking or answering it so…wtf?
Long press, nothing but share, delete, archive, or add to album. No sync, download or anything of the sort. Is immich supposing it's a one-way trip? What if I want a photo I backed up a year ago, I have to open the webui?
Maybe the feature is just not implemented yet?