Read more at the Official QOwnNotes documentation.
Use your ownCloud / Nextcloud sync client to sync your notes, your notes are simple markdown files. There is no need to invent the wheel twice.
And there are also people who are using different services to sync their note files (like Dropbox, Syncthing or Resilio Sync).
QOwnNotesAPI just handles the access to your trashed notes and note versions. Note sharing is done via the API provided by ownCloud / Nextcloud. Tasks are accessed directly via CalDAV.