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Viewpoint0232 I had the same issue: I was transferring the backup from an old phone (with Android 9) to a Pixel with GOS, and it didn't want to recognize the backup. I've written extensively about it here.
it didn't work by just copying the files in /Android/media/com.whatsapp/. It didn't offer me to restore the files.
That's the same problem I faced. I suspect that's because of how the file system works and the changes that were made from Android 10 onward (file-based encryption, etc.), but other messaging apps that had a local backup allowed me to transfer all the encrypted messages and the media flawlessly, so I assume that the programmer at Facebook/Whatsapp are a bunch of retards that don't know how to make even the most basic stuff work (also, have you looked at how outrageous some whatsapp functionalities are? Only disabled persons could come up with such poorly thought solutions).
Also, it's specified on the official WA website that from Android 10 onward they don't support the manual transfer of backups (really? the most straight-forward and flawless mode of backup is disabled by those douchebags at facebook...). So I can't understand the part where you say
I suspected this is because it didn't know how to decrypt them but no idea what was the real reason.
What's there to decrypt? Where would you specify a decryption key in the WA backup database, apart from the app itself when doing said backup?
In the end I was able to use the built-in transfer from one to another Android phone.
As also stated in my linked post, the chat transfer function did not work for me, because it is such a poor implementation that a toddler would know how to do that better, and does not work when the backup is too large or contains a lot o media.
I also wonder if it would be possible to do a Google Drive backup
Yes, that's the option that worked for me. And sadly, as many thing in our era, it's the most invasive and least secure and privacy-friendly.
I installed the play services and given them the minimal amount of permissions necessary. See my linked post for details, but it's a straightforward method: create a throw-away anonymous Google account, connect it on the old device, do the encrypted backup, then specify it on the new WA on GOS upon launching the app for the first time, and restore it. Then nuke everything, from the backup to the account itself (if you plan on not using it ever again, like me).
And after you nuke the account and uninstall all the Google spyware (services & store) the is no trace of it left, because thanks to the GOS developers all the apps are sand-boxed, and Big G does not have any privileged access to your phone, and no files remain after the uninstall (apart maybe for some temporary files). Also, WA's notification (even calls) function perfectly without google services, because they (apparently) have a double implementation, through google firebase and through their proprietary servers, and it switches automatically between them.
