How it works, and what it is not
The link carries your saved data, compressed, in the part after the hash. Nothing is sent to us; a link opened on another device unpacks it there. Send it to yourself on WhatsApp, save it in your notes, or keep the file. Make a new one whenever you have played a lot: the link is a photograph, not a live sync.
Merge is the safe choice when both devices have been played on: solved days from both are kept, the best streak and the best rating win. Replace makes this device a copy of the other one.
When the app comes, this same code will carry everything into it.