How it works: I don’t know about this service in particular, but usually the shared contains the encryption key so like this: example.com/files/file_id/encryption_key or something similar

As for trust: This appears to be a individual, so you will have to just trust it when using the public instance. However, since it is FOSS, you can audit the code and spin up your own instance

Privacy wise it very much is still Firefox with different defaults (telemetry disabled, do not track enabled), the changes to upstream are mainly the UI and some performance enhancements