I set up snyc on my devices on my second device it prompt me that there is a merge error. So i though ok it will override some cards… it override the hole data base decks that only exist on the first and not on the second device were deleted….. Why? Why does the data base deletes decks that weren’t effect by the conflict how do i get them back or are my decks on my first devices just gone?? 
Restore those from the backups: Backups - Anki Manual.
Conflict is between collections.
What is the difference between decks and collections?
In anki can only see a button to create a deck not a collection. So i assume all my decks belong to the same collection. Still the question why is the merge bulit like over writing whole collections and notnon a deck base? Because in that case you always over writing all your decks no matter on which you worked which leads to this kind of errors when i worknon different devices that are not always connected to the internet
One-way syncs happen only when you do certain operations like changing note type, modifying fields of a note type. (Apart from your first sync when you also need to do a one-way sync).
You’ll receive a warning for such operations so you can make sure everything is in sync beforehand. A good practice is to sync everytime you use Anki, when you open the app and when you close it.
Yes, every profile is a single collection. Getting Started - Anki Manual .
I am sure if there was an easy and better way to do it, one-way syncs would be gone.
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After you restore from a backup to get back what you lost – Restoring an Automatic Backup (Recovering from Data Loss) - Anki FAQs – follow these steps to merge the conflicting versions of your collection – Syncing with AnkiWeb - Anki Manual .