They're fundamentally different in the problems they solve. Dropbox is a cloud file storage and syncing service. NextCloud is an open source alternative to something like Dropbox in that it offers file sharing and syncing but also much more on top of that. It's really closer to something like the Google suite of personal cloud services with Google Drive, Photos, Contacts, and Calendar. Kopia is a backup solution for the files on your computer. You can use cloud file storage providers as the destination for these backups but it doesn't handle the storage of the backups itself. You have to provide that storage to it.