[Android Central Question]

I don't understand. I know that Smart Switch doesn't work with the Secure Folder, but I still made a backup inside the Secure Folder app. Yet when I restored it on my new phone, all the files were gone. When I checked on the File Viewer app, turns out all the files turned into .dm files. Does this mean that I've lost all those files forever?

[Rukbat]

Read this explanation of .dm files, then you'll understand what you're dealing with. Whether you can open them or not is another question.

General rule of thumb with computers (and your phone is a computer that happens to be able to make and take phone calls): When backing up protected or encrypted files, back up the unprotected or unencrypted version. You can always keep that on your PC or laptop and back up the protected or encrypted files to the cloud if you want. But backing up an encrypted or protected file in that state is dangerous - as you've found out.

Have you lost them forever? Read the page I linked to and see if you can recover them.