downloaded the linuxserver/syncthing and it worked but it syncs to /config folder rather than /sync folder. This is the main issue here
Guess I solved it, this how it's done: (every folder name \ other names are from my setup, though they are similar to TDL's videos)
In Docker Container window under "Volumes and Binds" section you set up two lines (see screenshot):
Then you go to Syncthing WebUI, click "Add folder", and put "istruggle" into "Folder label" field and "/sync/istruggle" into "Folder path" field, and click "Save" (see screenshot). No quotes.
Now you have folder /sharedfolders/General/Sync/istruggle at your OMV NAS and it can be shared and synced with other devices.
My error (and yours it seems) was to put "/sharedfolders/General/Sync" into "Folder path" field in Syncthing WebUI. However for docker you've bind "/sharedfolders/General/Sync" to simply "/sync".