How does sharing work?
The information required to decrypt an album is encrypted with the recipient's public key such that only they can decrypt them.
You can read more about this here.
In case of sharable links, the key to decrypt the album is appended by the client as a fragment to the URL, and is never sent to our servers.
Please note that only users on the paid plan are allowed to share albums. The receiver just needs a free ente account.