If you think "syncing" is making sure all your devices have the same ebooks on them... it doesn't work like that with the Kindle (or its apps). Get that outta your head.
All of your Amazon purchases are in your Archive List. You can access your Archive List from all of your devices (if you keep them all synced) and decide which books you want on which devices. But you must download each book onto each device yourself. Content. is. not. synced.
If they are non-Amazon purchases, you must side-load (or email) each individual book to each device. These won't ever be part of your Kindle Archive
"Syncing" will then maintain: (Obviously the Archive List) furthest read page, bookmarks, notes and highlights of the books each device has in common. This makes it easier to read the same book(s) on two or more different devices.
(NOTE: with non-Amazon books, the only thing you can hope to sync is the Furthest Read Page)
Clear as mud?