Network-level encryption is via TLS, comms level encryption uses the same Double Ratchet approach that Signal uses.
Lots more with threat models broken down here:

GitHub
simplexmq/protocol/overview-tjr.md at stable · simplex-chat/simplexmq
⚙️ SimpleXMQ - A reference implementation of the SimpleX Messaging Protocol for simplex queues over public networks. - simplex-chat/simplexmq