How to backup notes from stock Notes app? I everyday use Samsung Notes (in Samsung Galaxy Tab S3) for taking notes and audio from  College lectures. But how can I get all of my files and store them in my external HDD? 

I mean, there's no file inside tablet's storage, nor inside my SD card. Moreover, I wouldn't like to synchronize any of my notes and audios with Samsung Cloud or something.

How can I retreive my Samsung Notes files and audios?
I've never been a user of Samsung so I'm not sure how the stock Notes app works and in what format it saves files. This question indicates that notes can be backed up to PC from a Samsung phone using Kies as .ssm files and subsequently be opened using some media player (like VLC / Real Player) or S Note app.
This question addresses different methods to backup .memo or .smm or .spd files created by Samsung Memo app (if it's the same what you mentioned as Notes app). Memos can be shared/exported one by one in readable text files or otherwise transfer the memo files to PC.  

Now, app data is always saved in one of these 2 locations: /data/data/package-name or /sdcard/Android/data/package-name. Package name of Samsung Memo app is com.samsung.android.app.memo as mentioned here but you can check yours from App info. Accessing the former needs root access. Also an app with permission.WRITE_EXTERNAL_STORAGE can save files anywhere on /sdcard. Samsung Memo app version 1.0.4517634 app saves it's files to /sdcard/BeamMemo. For other versions, it can be different; /sdcard/.ActionMemo or /sdcard/SNote Data. You can search for these files using some File Explorer app or find command through commandline.  

Once the backup is done, you can either:


Restore these files back to phone later on. In this case you should backup the whole folders and not only the memo files. But still it's not guaranteed that the app will recover whole data if it is reinstalled unless a backup file(s) is created by the app for restore purpose. If you are restoring to /data/data/, app UID and SELinux context also need to be restored.
Open these files on PC. .memo files are basically zip files which can be extracted to get media files and text contents as answered here.


Or you can use a free app like ColorNote (for text-only notes) or other one which is easy to backup and restore.