Sandbox reset and import of package

I created a package that includes a schema in Sandbox A. After resetting the sandbox (which wiped all resources, including the schema) but not package, I re-imported the same package in  Sandbox A and, surprisingly, the schema was re-created successfully.

However, the package only references the schema URL/ID, not the full schema definition. This raises a few questions:

1. How is the schema being re-created just from the package, when the schema was wiped out with the sandbox reset?
2. Is there any underlying backup or storage of the schema outside the sandbox that supports this recreation?
3. If yes, where is that backup stored, and how long is it retained?
4. Can we rely on this behavior long-term for restoring schemas/resources after sandbox resets?

Would appreciate any insights. Thanks!