So first, no, all the files should not be accessible : There are special not "files", but keys, like the key used for this method. These keys pose a huge security risk of they are leaked somehow. The key can be something used to encrypt the device/disk, encrypt a connection, and other things associated with encryption.

And because of that security risk, they are often stored in a special chip or simulated chip (like the simulated tpm 2.0 on pc cpu), and not just "stored" so any malware or who knows what can access them just by reading the drive.

Second, the key is never transfered. When you connect to another device, that other device will get another key. Or maybe could it be backed up somehow in case of recovery on another phone? But that would defeat the entire purpose of this.

How Google can do to allow you to connect to another device if the first one is lost, not sure. But it would certainly either ask for a password and a 2fa method.

Keys are stored in the equivalent of iOS’s Secure Enclave (actual name is implementation specific: ARM’s TrustZone, Samsung’s KNOX, Pixel’s Titan M, etc): https://www.howtogeek.com/387934/your-smartphone-has-a-special-security-chip.-heres-how-it-works/

I just replaced my iPhone, and the few places I “sign in with Apple” still work in the new phone. Yes, you can back it up and restore to a different device. I assume you can also use it across devices but I haven’t tried that

And it's expected as you still had that device. And it's not the same key, a new key has been created for that new device. Now if that device cannot be accessed?