does there exist an app that can sync to iCloud notes? Is there any app,
That iCloud notes can sync to ?
I wanted to have a backup of my iCloud notes
I know iCloud is already supposed to be a backup, but I have heard of people having problems or notes being deleted out of nowhere, so that's why I wanted another kind of backup
That could sync to iCloud so I could keep my notes on iCloud, plus somewhere else
Does such a thing exist ?
Thank you.
Firstly iCloud is not a backup.  It is a synchronisation tool.  Hence the problems you have heard about with notes being accidentally deleted and then gone for ever.
So you need a backup.
But this is not so easy because the notes in Apple Notes are stored in a database and not as separate files.
One solution is to periodically export your notes out of Apple Notes into separate files in another format and store them somewhere included in your regular backups.  For this get the app Exporter from the Mac App Store.  But this is a one way operation - the only way to import into Apple Notes is by copy and paste of text.
The problem you have highlighted is in common with other popular note taking applications (e.g. Bear) which store notes in a database and synchronise with iCloud.  Backup is not straightforward.
For myself, I have given up using apple Notes for anything I want to keep.  I now use Typora and have learnt to use markdown for formatting.  Each note is a separate file and backups become easier to manage.  But I have lost Notes' ease of use and synchronisation to iPhone.
Edit:
It is possible to recover lost or accidentally deleted notes.
The Apple Notes database (and other stuff) is located in ~/Library/Group Containers/group.com.apple.notes.  This is part of the Time Machine backup.  So there is potential to recover lost notes by using the content of that folder from a Time Machine backup.
Here is a process which I tested and works:

Delete an unimportant note (to simulate the loss of an important note!),
Exit Notes,
Remove Notes from iCloud (in System Preferences - Internet Accounts),
Recover the folder from Time Machine using drag and drop in Finder,
Start Notes.

Notes shows all the notes that were present at the point in time when the TM backup was done.  But it is not connected to iCloud and so can't merge the recovered database with any differences in iCloud.
Now:

In Notes, move the recovered note from iCloud folder to the &quot;On My Mac&quot; folder,
Exit Notes,
Reconnect Apple Notes to iCloud (in System Preferences - Internet Accounts),
Start Notes.

Notes merges Notes' iCloud folder with the content on iCloud, but leaves alone the recovered note which is in &quot;On My Mac&quot;.  If desired this can be moved back into one of Notes' iCloud folders.
This is not a straight forward process and is open to error and the risk of losing all notes.  But it is possible if you need to recover an important note.