ChaCha20-Poly1305

This key pair alongside the destination's converted ED25519 key pair is used to generate a shared symmetric encryption key.

The payload is then encrypted with the shared symmetric key using either AES or the XCHACHA20POLY1305 algorithm, a variant of the widely adopted CHACHA20POLY1305 cipher suite.

And so this kind of tells you how they're generating two.

They're generating two key pairs and then they're signing stuff with it.