This is the scenario: I did a factory reset on my windows 10 PC, installed my needed applications including Joplin. I'm using Joplin before and it is sync using OneDrive. Now, I wanted all my notes back so I use again the sync with OneDrive but I think it only sync these few welcome notes and not my actual notes that is stored on OneDrive. How do I fix this?
Did you backup your profile directory . . . \.config\joplin-desktop and then use it to replace the profile directory that new install of Joplin created?
Sounds like you did not. So as far as that Joplin install is concerned, this is a fresh install, which is why you have the few welcome notes.
Do you have a backup of your note data either via profile directory or JEX files, or is the sync directory on OneDrive your only copy of your note data? Is it still there?
Surely it should be the same as if one was setting up a new client though. i.e. if I install a new instance of Joplin on a different PC it will give me the welcome notes, I add the sync target, sync and it will pull all the sync data from the target + upload those welcome notes.
JEX files are the lossless file exports from Joplin (Joplin Export format). More info here
The profile directory is basically where Joplin 'lives'. It has the main database along with your note resources and other data. You can find it in the .config/joplin-desktop folder. As you are on Windows it will be in C:\Users\USERNAME.config\joplin-desktop.