Read more at the Official QOwnNotes documentation.
QOwnNotes works with simple markdown files. You can sync them with any file syncing solution you like to any other device.
Most popular would be to use a private Nextcloud or ownCloud server and sync them with your Nextcloud or ownCloud sync client to your server. Of course other software, like Dropbox, Syncthing, Seafile or BitTorrent Sync can be used too.
To access your Nextcloud / ownCloud notes from your mobile device you may want to get one of these apps:
On Android you could also use any sync-tool like Synchronize Ultimate or FolderSync to sync your note files and use software like neutriNotes to edit your notes.
On iOS I heard Notebooks works well (syncing notes via WebDAV).