The transport security used in QUIC is using TLS 1.3 (RFC 8446) and there are never any unencrypted QUIC connections.
TLS 1.3 has several advantages compared to older TLS versions but a primary reason for using it in QUIC is that 1.3 changed the handshake to require fewer roundtrips. It reduces protocol latency.
The Google legacy version of QUIC used a custom crypto.
