Take a look at the documentation of Lemmy.

The server-to-server communication uses the ActivityPub protocol

For backup you can look into rclone. It’s what TrueNAS is using. You can set rclone up to only upload your data encrypted to a lot of storage providers like BackBlaze, Google, Dropbox, S3 etc. So you don’t have to trust the provider with the privacy confidentiality of your data.