What data does Join encrypt when encryption is enabled?

Join encrypts any data that is not required for the system to work directly.

For example, when you send a push, the recipient of the push is not encrypted because Join wouldn’t be able to tell which device should receive the push.

On the other hand, when you send a push with some text, the text will be encrypted because the System doesn’t need to know about it to work correctly.

Everything you upload to Google Drive is not encrypted by Join but is actually encrypted on the Google Drive Server.
The reason you can read it is because you are authorized to login/access that Google Drive account. Therefore it is “clear text” to you. On the actual Google server, it’s encrypted.
Everything on all of the major cloud providers (Google Drive, Dropbox, etc) is encrypted. It’s decrypted by the “client software” (or the Web App) when an authorized user accesses it.